Pee Pee

We have a pet fish named Pee Pee... He's a Beta. He is in memory of our other roommate Katie... also known as Pee Pee.
Something is wrong with our fish tank and there is brown fuzzy stuff growing on the side of the tank. The brown stuff is overtaking the tank. I wonder if we leave it long enough it will eat Pee Pee up.

I've changed the water a couple times but it doesn't seem to help. Within days the brown stuff is back. I think there is a disease in the tank. Perhaps if I change the rocks....
I don't know what else to do.