I've been scratched by coral and had the same problem. It could be that the scrapes are just infected, which means some antibiotic cream or ointment should help the healing. Or, it could be a reaction to the coral's stinging cells. Some (maybe all?) corals have nematocysts, which are stingings cells designed to inject venom into prey (tiny amounts, but still could cause a reaction in humans). It's like getting stung by a jellyfish, and like jellyfish there's varying degrees of how much it stings. If the antibiotic cream doesn't do the trick, I'd go to the doctor.