You need to review your system... Algae on pic shows nutrients and stability could be off.
Besides that there is lots of params that you should keep a close eye when having a zoa tank.
Small tanks aren't that good for ANY marine organism because they will help those ups and downs on the params.
It's not aways a hard to keep specimen, but ignoring that they are struggling to survive having only 3 or 4 polyps to begin with!!!
The way people sell them nowadays is just ridiculous. It makes harder and harder to keep those abused tiny frags!!
Goos luck!
Grandis.