Ugh - mouse crud

What the heck do I call this crud that accumulates on the feet of my mouse? It looks like it's a combination of dust, lint, and secret sauce in a tacky layer that makes the mouse drag instead of glide across my desk. Sometimes I can scratch it off with my fingernails, other times I have to use my car keys or something hard and sharp.

I think I'll call it "MOUD," short for "MOuse mUD."

What-EVER.