can't tell if scratch or bite

September

so yesterday evening we noticed this on our babies arm. i was thumbing he couldve scratched himself because his nails grow fast and sharp still like he can defiantly cut with them but it looks a bit weird.. pictures aren't as clear because hes 5 months and won't stop moving his arms. not sure how to go about cleaning if or what to do. i just used water so far and soap when i took him a bath.