Sleeper Goby dwindling in health?

Hi there! Recently I purchased a mated pair of sleeper or maiden gobies at the lfs. Recently one of them died (I woke up and found the fish lying next to a piece of live rock with a hole in the bottom of it's stomach.) Now the other goby is very skinny, even though I watch her eat daily. Is there a reason why this could be happening? I'm feeding her a diet of brine shrimp and algae. No sand in tank. Any help is appreciated, thanks!

size of tank is 55g. other inhabitants include 2 clowns, 1 splendid dottyback, 2 urchins, 5 damsels, cleaner wrasse, chocolate chip sea star, brittle star, cleaner shrimp, and coral banded shrimp. lots of liverock (the dottyback loves playing hide and seek).