Remember even corals will die away only to return when the light does.
I don’t have personal experience with these guys in my tanks at home; though I have seen them before at some LFS. I would follow Al’s original recommendations vs light. Ken put rocks in the dark for a year to kill algae.(Didn’t work.) Also many of these animals might be mainly dependent on light, but can get energy in other manners. Also think about a chocolate chip star fish or some urchins that take up to a year or more to die slowly of starvation or mal nutrition. Removing one energy source isn’t the way to go if you really want to remove a past. Manual removal as much as possible and then to weaken chance of respawn you can try depleting their energy sources.(reduced light and nutrients/increased water changes)
Just an update these guys all disappeared, I’m 99% sure they were hydroids as the same time they disappeared their larval “snowflake” stage stopped showing up which I would see every now and then. I didn’t do much, just added more corals and they seemed to disappear or get out competed or my pods got tired of them and wiped 'em out or my Dory ate them.