Parameswara has Five-Fold functions as I discuss in my answer here and also referred in 5th Anuvaka of Sri Rudram. The verse discussed in Saundarya Lahiri is:
जगत्सूते धाता हरिरवति रुद्रः क्षपयते
तिरस्कुर्व-न्नेतत् स्वमपि वपु-रीश-स्तिरयति |
सदा पूर्वः सर्वं तदिद मनुगृह्णाति च शिव-
स्तवाङ्ञा मलम्ब्य क्षणचलितयो र्भ्रूलतिकयोः || २४ ||
jagatsūte dhātā hariravati rudraḥ kṣapayate
tiraskurva-nnetat svamapi vapu-rīśa-stirayati |
sadā pūrvaḥ sarvaṃ tadida manugṛhṇāti ca śiva-
stavāṅñā malambya kṣaṇacalitayo rbhrūlatikayoḥ || 24 ||
Dhata begets the world. Hari protects (it). Rudra destroys (it). Isa withdrawing this (Dhata, Hari and Rudra), obscures even his own body (by withdrawing into Sadasiva). Indeed Siva (whose name) is preceded by (the word) Sada, obliges in all this, stayed by your order, exhibited by the momentary movement of (your) eyebrows.
The Sanskrit word used is 'Isha'.
The verse recollects five forms of Parameswara doing five-fold activity. Creation (Shristi) is done by Brahma, Protection (Sthiti) is done by Vishnu, Destruction (Laya) is done by Rudra, Obscuring (Tirodhana) is done by Maheswara and Gracing (Anugraham) is done by SadaShiva.
So, it's clear Isha there is referring to Maheswara/Ishwara form among PanchaBrahman. As told in Shiva Purana:
ब्रह्मा विष्णुस्तथा रुद्रो महेशानः सदाशिव ।
मूर्त्तस्तस्य विज्ञेया याभिर्विश्वमिदं ततम् ।।
ईशानः पुरुषोऽघोरो वामः सद्यस्तथैव च ।
ब्राहाण्येतानि देवस्य मूर्तय पञ्च विश्रुता ।।
His forms comprise of Brahma, Visnu, Rudra, Mahesa and SadaShiva which pervade in the universe. The well known images of Shiva comprise of Ishana, (Tat)Purusa, Aghora, Vamadeva and Sadyojata.
Maheswara corresponds to Tatpurusha swarupa among Five Heads of SadaShiva.