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I wouldn't use a blowtorch. The water within the live rock will turn to steam, then the rock will explode in your face.

I have a small patch of hydroids which not by coincidence is right under my feeding area. IME they grow with excess nutrients and recede when nutrient level go down.

I find the suction hose/filter sock in the sump trick while trimming the hyroids with scissors keeps them in check. I have some Aiptasia-X but I haven't tried it on my hydroids yet.