Looks like planaria, pretty harmless (as in not muncing on your corals) but they do reproduce pretty fast and fill up the tank. Can even cover soft corals and LPS to the point where they dont get any light.
I had a couple of nasty struggles with them in the past and when they are in the hundreds killing them is risky as they release a toxin when they die that can kill your fish.
Did you just get the macroalga? if yes then they haven't reached your tank yet most probably, I would treat the sump only.
Any fish can be hit or miss with those, mandarins sixlines, Halichoeres wrasses like Melanurus have been reported to eat those but its always hit or miss... Would suggest blowing a few in the water and seeing if the fish you get will eat them or not.