First, do not let it get that dirty. Clean more often.
But if this has happened, letting soak in a super soaker overnight is a good way to let the resin fall off.
You can also rinse under hot water. The hot water will make the resin softer, and the pressure will push the resin out. It is good to not let the water get too hot or you could burn yourself. Only go as hot as you can handle.
Using a pipe cleaner will also make your cleaning job easier.