Charcoal stain on moleskin pants

That’s a physical stain, rather than a chemical one. That means there’s not easy fix. If you have a steam cleaner, blast it and that will get it out eventually. Failing that, squirt some dishwashing detergent on it, rub it in with your fingers for a while, then rinse and repeat. It’ll come out eventually.
Something else that may work is to put a paper towel on it, then run a steam iron over it and blast it with some steam. It may wick up into the paper towel