They are sensitive to the water perameters. What are your specs? Also they love shelled inverts. Any in the tank? Choco also will take out sleeping fish. Any missing? He could have eaten something that did not agree or the water is not perfect or there is a fish torturing him to make him throw his guts up. Pick an option.
They evert their stomach to feed. If this is all it is, then it is normal and fine. But flakes are really not sufficient. Feed meatier stuff and diverse things. Krill is good, but add shrimp, squid, silversides and even algae wafers or nori.
does it eat coralline and copepods? Cause it only does it on the glass where there is coralline algae and maybe some pods. Do i just have a weird starfish?
It may be eating "surface films" meaning algae and bacteria and microscopic critters on the glass and other surfaces. Still, it is a good idea to give it something more filling