Just a quick update as I was going through the same problem. I added a generic sea hare (Dolabella auricularia) and so far about 90% of them are gone. Actually watched it eating them. There are still some on some of the rockwork where I know it hasn't been as I have a small amount of algae on those rocks also and trust me this thing if it gets to the algae it will eat it. Absolute beast of a CUC soon as the tank is clean I will probably move it to the sump and let it clean all of that and then wait and see if any come back as its the hidden ones that are the problem.