Possibly shrimp. And imo, re-evaluate the rest of your cuc as well. Although they may not eats the adults, many species will eat the young spores. Like peppermint shrimp with aptasia. I've noted a similar thing with too many asterina and my stomatella population.
One way to id a tunicate is to look very closely in still water for the siphons. You should be able to see them on owning and closing their mouth.
If it is a tunicate you may need to put the tank on a diet. No aminos , rinse your food etc. they are filter feeders. So a combo of both predators and diet.
If it's an algae spreading that fast, you may need to consider a nuclear option like vibrant.
One way to id a tunicate is to look very closely in still water for the siphons. You should be able to see them on owning and closing their mouth.
If it is a tunicate you may need to put the tank on a diet. No aminos , rinse your food etc. they are filter feeders. So a combo of both predators and diet.
If it's an algae spreading that fast, you may need to consider a nuclear option like vibrant.