Sea Hare appears to have chunk of skin missing

[Octavarium]

I purchased my sea hare a few weeks ago because of an excess amount of hair algae in my 28 reef....plenty to eat. However lately I've noticed it appears his skin is "Wearing away" on his back, near the 2 holes...and my lawnmower blenny was plucking at it! I immeadiately moved him to my 29g with same temp/salinity that only has a small orange spotted goby, red star, and gold stripe clown... and yes plenty of algae to last him months. My question is what could have caused that? Was it the blenny attacking him?? I never read anywhere of it happening when it googled. Readings on both tanks are monia/trites/trates 0. SG 1.025, 78 F, RO/DI water only.

 

[Elysia]

It is not a good idea to keep sea hares or lettuce seaslugs with animals that may pick at them. Many of these animals need to keep their skin entire inorder to stay alive.

Algae blennies are notorious nippers, and not just of algae or things that appear to be algae. It is recommended that algae blennies not be kept with peaceful species such as seahorses because of the blenny's attitude.

Some seahares species do not have a purple gland, and those that do will be unable to release the colored "ink" when molested if they have not eaten red algae lately.