STOP!! Do NOT use anything on your hull if it is painted...you will need an entirely new paint job if you use anything with any abrasive qualities or even a soft scrub type sponge.
If you have a scrape from the dock that goes into the paint...there is nothing to do but get touch up paint and repair.
If instead what you have is rubber from the white dock bumpers this can eventually be removed with a soft cloth or brush and dishwashing liquid. I have found (don't ask me how!) that the white rubber actually degrades quickly and is easier to remove if you just wait a week or so.
If you must DO something now...try the acetone Zan suggested very gingerly on a little bit of it and with a soft cloth.
No longer posting. Reach me by PM!