What be this on my chromis

[Dearth]

Noticed this on one of my Chromis yesterday been fairly busy so really haven't looked at tank in last 6 days or so. It has got what appears to be a cyst or a boil on its side with a hole at the top of the cyst/boil. The fish has not been from what I can see in distress nor has it been swimming erratically from all appearances been doing its normal fish stuff.

Haven't been able to find anything online so can anyone tell me what it might be

Here are 2 pictures sorry taken with my IPad not best quality