Photo is so fuzzy I can't really see much other than some redness. Is there visible damage (like a cut, lesion, etc)?
What makes you think a crab grabbed it? If this is something you saw, or is a habit, you need to lose the crab, or relegate it to "sump monkey" status. What kind of crab do you have?
That being said, redness usually means that a wound is infected, which means you should treat it with a broad spectrum antibiotic. However, I'd like to see the affected area better before suggesting treatment.