what algae are you speaking of? if you have some algae on the rocks, when the chaeto blocks the light from above, it will disappear. i'm going to give you advice, but i don't follow it. here it is- you shouldn't have any predatory animals in the refugium. it's supposed to be a place free of predation where the innocent little beasties the rest of the tank use for food, can grow and multiply.
i have all kinds of killer beasts in my refugium...mainly because i don't have anywhere else to put them. i have a large green brittle star, and multiple gorilla crabs.
the gorilla crabs came in on different corals from time to time, and i hate to kill them, so i toss them in there.
i haven't seen a single one since i tossed them in, so maybe they die without sps to feed on?
meanwhile, if i was going to choose something to be in the refugium, it wouldn't be something that eats algae.