My fenders did not get goo on them. They would get a black graphite-looking material that came off the black plastic dock floats, and upon rolling along the hull they would transfer the material to the hull. It would all come off with fiberglass polish, which I would use on both the hull and the fenders to take the color off.
To proactively prevent it, I made my own fender covers with old T-shirts. I call them my "trailer trash" fender covers. By the end of the season they look disgusting, but my fenders and hull look pristine. Each year I remove the old one and put a new one on:
Note the great shine on my freshly polished hull:
That's funny . I did that once, put t shirts on them . My friend came by with a felt marker and put eyeballs on them . Lil' ghosties .