I sometimes get these on my bigger cichlids and discus. eventually it will pop like a real pimple and will go away, when it pops is when you need to watch for infection. sorry to say this but its a sign of poor water conditions. IF in fact this is what it is. its hard to tell from the picture but if its just a pimple and they normally happen around the fins or on the body near the edge they just come and go no need to medicate, maybe do a couple big water changes back to back
Quite a few things it could be, most disease or parasite-related.
Cancer, lymphocystis, infections from wounds, and various external parasites can all cause cysts like that.
Personally, I would quarantine the fish in order to keep a better eye on it, see if you can identify a possible treatment, and possibly help prevent spread through the rest of your tank in case it's something contagious.
I just lost a gold ram to that stuff. It had been coming and going for about a month or 2. It never looked bad so I just thought it was going through a phase, but it got worse and upgraded to popeye so I put it in QT. I ran the temp at about 86 degrees and dosed tetracycline.
It didn't work so you can rule that out as an option?