I don't mean to go against the vet but cysts are normally fluid filled sacks. That looks like more than fluid. Just monitor it and if it goes white in the middle (like a pimple) and then red around the edges, it is an ulcer.
You could try to pop it with a needle and drain the gunk out. That way you prevent it popping in the tank and possibly releasing nasty stuff in the water. If you do this make sure you use a sterile needle and disinfect the area before and after. Do lots of big water changes to keep the number of microscopic organisms down while it heals.
