chammer..dont know if i fully understand the ? your outside brick shouldnt be on drywall. if your looking from the inside through the stud wall it should be plywood and if its moldy that meens water is getting in between the brick and plywood.
first you need to make sure that the exterior plywood is structurally sound if it is you need to seal where the brick and the window meet with a GOOD exterior caulk that will prevent any future water from entering. if it is drywall it needs to be removed and plywood or particle board put in its place.spray the mold with Clorox that will kill the mold