Sorry, it's Yiddish for "dirt" or something similar.

Anyway, don't scrub hard with the toothbrush, that might break the hooks -- short strokes in the same direction should at least get the wool (or whatever) fibers sticking up so you can pull them out with your fingers or with tweezers. But if it's tangled up on the "woolly" side of the Velcro, I'm not sure how to fix that!