My guinea pig had a bald patch and dry skin around his eye.?

I keep my guinea pig Angus inside my room to avoid mites and fleas. I left him for a couple of days and when I came back, Angus had this bald patch and dry skin around his left eye. What is the cause of this? He only eats pellets, natural hay and sumtimes green bell pepper. I'm going to bring Angus to a vet tomorrow. Would you mind giving me a heads up? And if it is healed, will the hair grow back? Thanks.