Sick Fish?

[MarkDarnell]

New to the hobby. What is this dark spot on green chromis? I had 7 of these guys, 1 has disappeared, and now this one has this spot. He is struggling to swim also. I have 2 clowns and rest of the chromis are doing fine. Any suggestions?

 

[ChristopherKriens]

Likely Uronema marinum..

 

[MarkDarnell]

Should I take him out immediately and put in a quarantine tank?