I'm pretty sure cyanobacteria is not commonly referred to and cotton candy algae. Perhaps I'm mistaken but I was under the impression cotton candy was a type of filamentous (hair) algae.
If you dealing with cyano then you have a nutrient or circulation issue. I still get a little cyano every now and then in some low flow spots even though my PO4 and NO3 are both undetectable. If your just getting and few small patches then its no biggie, just blast it with some flow every now and then to dislodge it or potion a powerhead at the low flow area where it is growing.
If you have cyano throughout the whole tank that comes back repeatedly then you definitely have a nutrient issue and you need to rectify it immediately
I don't know of any species of sea hare that will eat Lobophora or cyanobacteria. They are however, IME, very good at eating green filamentous algae like green hair. However, there are several species so I will try not to make broad generalizations. I have kept some the the larger Pacific Dolobella species without much success. They would just disappear in the rock work never to be seen again. I purchase a smaller seahare of an unknown species from GWA a few months ago and it does a great job and eating green slime. I don't really have any hair algae issues and it doesn't touch my Lobophora.
Now Lobophora is something I have alot of experience with. If your tank can support it, just get a small Naso tang. They will destroy it, as they are brown algae specialists, most fish and inverts are not. I have had limited success with using Diadema (black long spine) urchins. Again, there are several species with in the genus, and I don't think all of them relish it as much as others but the one I have has put a dent in the stuff.
Good luck!
