I'd say it looks like a gear tooth. How much oil is it dropping? To weld it up you'll have to drop your gearbox oil again, weld then refill and hope the weld takes. Other option, if its only weaping a bit is to clean the cracked area with wax and grease remover, give it a light sand then put a smear os Sikaflex over the crack.
Oh and i'd be getting a new drain plug to, that one is well mangled