Size of tank in gallons or litres.
How many fish and which type.
Water stats in ammonia, nitrite, nitrate, and ph.
Does the lump look soft like filled with fluid or hard looking.
any chance of a pic?
I gather the shop didn't tell you how big they get. Fire eels can reach about 2ft long and when mature will eat small narrow fish like neons and guppies.
The tank is also a bit small for a common plecostomus to be kept in permanently. It will be fine for a while but will need a bigger home eventually.
Make sure the water qualit yis good and if you have to treat the ell, be careful about how much and what sort of medication you use. Eels are scaleless fish that don't tolerate medications very well.
small environments will slow the growth of fish so they might not grow very quickly.
without seeing the fish it is hard to say but it could well be damage caused when the fish was caught. If you keep the water clean then any minor infections should clear up without the need for any medication. If the area goes red and inflamed then it is infected with bacteria. If it goes white and fluffy then it has a fungal infection. But give it a couple of days and see how it looks. Most minor injuries heal up quickly.