Tortoise problem

I found a tortoise about 2 years ago on my front lawn. I brought him to a vet who said he was about 15 years old. I kept him...there is a heater in my 45 gallon tank. He sleeps a lot but also hangs out in his water dish, he eats, and he is active, But i notice crust on the sides of his face by his mouth..and his eyes struggle to open. What could be the cause of this and what can I put on his skin to make the crust go away?