Parasitic monogenean trematode. The parasite is translucent and hard to see. Quite common in marine aquaria. There are few species of Benedenia Trematodes but all have a similar set of symptoms, and the treatment and prognosis are very similar.
Seldom fatal as it takes a long time to develop. If recognized the outlook is good.
This malady is most often observed in Public and home aquaria. It is best when purchasing new specimens to watch them for a few days to see they are clear of any obvious symptoms. If need be offer the vendor a deposit and say you will collect fish after a week.
From June 2013 https://doi.org.1371/journal.pone.0064987
Akiyuki Ozaki et al.