First, I'd try the mild soap you use on your topsides with some warmish water, paper towels or sponge. Then, I'might try a Inflatable cleaner (you could try West Marine for the various products.
Finally, if it was really and issue -- there are more agressive cleaners; but, I'm not sure what it would do to the Hypalon. If you keep the raft inflated and in the sun a lot (which you apparently do because otherwise you wouldn't have put antifouling on your tender?), then I'd probably put one of the treatments on the Hypalon to help with blocking UV and making cleaning easier.