In the book Yoga Vashistha(page 426) , Vashistha says one should give up moha as the snake casts off the skin.
My question is : should one give up moha and what is the meaning of the word moha ?
Yes, would be good to give up moha. Patanjali uses the word in YS II.34. Patanjali is an extensively commented author. It is usually understood as blind attachment, delusion. He names moha as one of the causes to "perverse, unwholesome, troublesome, or deviant thoughts", what is obviously an serious obstacle to your practice.
Bhagavad Gita speaks about the dangers of delusion (moha).
Deluded by the mental states accruing from the three Gunas of Prakrti, this world knows not Me, the Imperishable, transcending these Gunas.
Niyatasya tu samnyasah karmano nopapadyate mohat tasya parityagas tamasah parikirtitah
It is not at all proper to renounce works that ought to be done as duty. Their abandonment out of delusion is considered to be of the nature of Tamas.
The word mohat means out of delusion. Finally what are these delusions brought in by our mind?
The mind brings before us all our delusions — body, sex, creed, caste, bondage; so we have to tell the truth to the mind incessantly, until it is made to realise it. Our real nature is all bliss, and all the pleasure we know is but a reflection, an atom, of that bliss we get from touching our real nature. That is beyond both pleasure and pain. It is the "witness" of the universe, the unchanging reader before whom turn the leaves of the book of life.
The Complete Works of Swami Vivekananda, Volume 8, Lectures and Discourses, Discourses on Jnana-Yoga - II
यदा ते मोहकलिलं बुद्धिर्व्यतितरिष्यति ।
तदा गन्तासि निर्वेदं श्रोतव्यस्य श्रुतस्य च ॥
—Bhagavad Gītā 2.52
thicket, heap, confusionbuddhir
intelligence, enlightenment, mental determinationvyatitariṣyati
it shall pass beyond, it shall cross over,
thou shalt gonirvedaṁ
of that which is to be heardśrutasya
of the heardca
When thy intelligence shall cross beyond the whirl of delusion,
then shalt thou become indifferent to Scripture heard or that which thou hast yet to hear.
When your understanding passes beyond the swamp of delusion,
you will be indifferent to all that is heard in Sacred Lore.
Once your buddhi has crossed beyond the snares of temptation,
you shall have become indifferent to the Scriptures you have yet to hear and those which you have heard.
The above verses do not instruct us to shed mōha, but speak of that shedding as an inevitable future. Confusion & ignorance should indeed be shed. However, do not try to remove delusions. They will fall away naturally through practice. If you aim to shed confusing, then you are acting with your focus on the fruit of the actions.
Just as a snake needs its skin for a time, certain delusions are useful for learning about a particular aspect of Self. Once a misconception is no longer useful, it falls away naturally, just as natural as a snake shedding skin.
There have been times I didn't realize I had shed a delusion until I saw another experiencing it, thinking, "Oh, I remember that."
क्रोधाद्भवति सम्मोहः संमोहात्स्मृतिविभ्रमः ।
स्मृतिभ्रंशाद् बुद्धिनाशो बुद्धिनाशात्प्रणश्यति ॥
—Bhagavad Gītā 2.63
it comes to besaṁmohaḥ
from delusion, from confusionsmṛti
remembered wisdom, memoryvibhramaḥ
from memory wandering awaybuddhi
from destruction of intelligencepraṇaśyati
one is lost, he is destroyed
Anger leads to bewilderment, from bewilderment comes loss of memory;
and by that the intelligence is destroyed; from destruction of intelligence he perishes.
From anger comes confusion, from confusion memory lapses,
from broken memory understanding is lost, from loss of understanding he is ruined.
Anger leads to confusion and the confused forget wisdom,
forgotten wisdom leads to destruction of buddhi and those without buddhi are lost.
From this line we can tell that wisdom is our natural state, but confusion—mohā—leads to forgetting that wisdom. By reducing anger, confusion is reduced as a byproduct.
इच्छाद्वेषसमुत्थेन द्वन्द्वमोहेन भारत ।
सर्वभूतानि सम्मोहं सर्गे यान्ति परन्तप ॥
—Bhagavad Gītā 7.27
icchādveṣasamutthena dvandvamohena bhārata,
sarvabhūtāni saṁmohaṁ sarge yānti paraṁtapa.
By the delusion of the dualities which arises from wish and disliking,
O Bharata, all existences in the creation are led into bewilderment.
—Sri Aurobindo’s Interpretation
All creatures are bewildered at birth by the delusion
of opposing dualities that arise from desire and hatred.
Because of the arising of desire and hatred—because of the deluding (power) of the opposites, Arjuna,
All beings fall into delusion at birth.
By the welling up of desire and aversion—by the delusion of dvaṃdvá—Child of Bharata,
at birth, all beings fall into delusion, oh Scorcher of Foes.
Because of the illusion of duality, all are born into mōha. So, we have forgotten self-wisdom before we incarnated here. Once released of dvaṃdvá, mōha is gone (BG. 15.5). But having it, we are in a hell of attachment (BG. 16.16).
तद्विद्धि प्रणिपातेन परिप्रश्नेन सेवया ।
उपदेक्ष्यन्ति ते ज्ञानं ज्ञानिनस्तत्त्वदर्शिनः ॥
यज्ज्ञात्वा न पुनर्मोहमेवं यास्यसि पाण्डव ।
येन भूतान्यशेषाणि द्रक्ष्यस्यात्मन्यथो मयि ॥
—Bhagavad Gītā 4.34-35
tadviddhi praṇipātena paripraśnena sevayā,
upadekṣyanti te jñānaṁ jñāninastattvadarśinaḥ
yajjñātvā na punarmohamevaṁ yāsyasi pāṇḍava,
yena bhūtānyaśeṣāṇi drakṣyasyātmanyatho mayi.
Learn that by worshipping the feet of the teacher, by questioning and by service; the men of knowledge who have seen (not those who know merely by the intellect) the true principles of things, will instruct thee in knowledge.
Possessing that knowledge thou shalt not fall again into the mind’s ignorance, O Pandava; for by this, thou shalt see all existences without exception in the Self, then in Me.
—Sri Aurobindo’s Interpretation
Know it by humble submission, by asking questions, and by service;
wise men who see reality will give you knowledge.
Arjuna, when you have realized this, you will not descend into delusion again;
knowledge will let you see creatures within yourself and so in me.
— B. Miller
Know this! By humble prostration, by enquiry, by service;
Those who know themselves—who perceive tattva—will point it out;
Thus having self-knowledge, never again shall you fall into delusion, Child of Pāṇḍu;
with it you shall behold all beings in your ātmán and then in me.
There can be no mōha for those who see all others in the self and the self in all others. The Bhagavad Gītā mentions several times that mōha is born of tamas (BG. 14.8, 14.13, 14.17, 18.7, 18.39, )