The photo is grainy and unclear. Looks like merely a coloration. Odd. Never seen this before but can't offer anything else at the face value of the info and photo given.
My tank is pretty bare other than the 3D background I have. It’s one with polystyrene and cement coating so I’m unsure if this could be a cause. They both appear to be acting as normal but I was originally thinking it may be some form of bruising. Thought I’d double check on here. I can try get a better photo if anyone thinks It may be something more serious. Thanks for the help
Also just to add. Both my giraffes are around 8 inches now. Earlier I noticed what seemed like breeding behaviour as one catfish was aggressively chasing the other towards its tail and then afterwards they were next to each other and one started aggressively shaking next to the other, almost like a fit. I thought they wee too young to breed but is there any chance it could be? Thanks again