Hard to tell from the pic but it could be a wound from 1 of the stones, they really prefer sand or very fine gravel. Do you have a water test kit? Any other tank mates?
Never kept a silver aro but my jar would give a nip to anything that come up to the surface, if not that I would maybe get smoother substrate but for now I would keep the water clean and keep an eye on it, if your worried about it then melafix will help keep him calm and protect from infection, good luck