welcome! that coin may clean up nice. give it a soak in distilled water for however long you like (hours, days, whatever), hit it with a toothbrush and toothpick see if anything starts to yield. if that gunk is dirt that will soften, piece will come off pretty quickly. if it is a harder type deposit, maybe not...or you may have to try something else if you want to clean it. but with distilled water, a toothbrush, and a tooth pick...you're not going to hurt anthing...so give it a shot and keep us posted!