If you can fit your hand in there, reach in there with a rag and wipe it up. The cleaner the better, but that's why there are two filters in the setup. Someone else was thinking about hosing it out, I suggested turning it upside down to do so, IF the tank is made so that the crud would actually flow out and not get caught in the sender fitting ring. Water won't hurt anything as long as it's allowed to dry thoroughly.