If it rubs off easily, that sounds like an ablative paint. Not something you can apply VC-17 over. Is it soft and chalky? If so, it's ablative, and you need to remove it completely before applying VC-17, but you can apply most other ablative paints over it. VC-17 seems to be the popular standard here in the Great Lakes, but lots of people use ablative paints too. What did you pick up at WM?
West Marine Advisor has a good chart that shows bottom paint compatibility for just about any combination you can come up with.