nasty canvas surprise

we popped the top on our '81 westy for a lake trip the other day and
got a nasty surprise. the canvas on one side was streaked with grime and
some mold spots. it appears a clamp on our outside awning has been
gouging a small hole in the side of the fiberglass pop-top, breaking the
rubber seal that keeps it from leaking and allowing several months of
rain to leak through the half-dime-size hole and onto the canvas. any
recommended product to get the stuff off? or are we looking at a big
patch or a new canvas job? thanks any opinions...