It is hard to tell if that is mineralized dirt or the
red encrustations that are a by-product of
bronze disease. I can't be certain from the photo but maybe others can.
The problem is that if it is hard dirt then there might be a nice surface underneath but if it is the dreaded
red crust then there might be horrible
pitting underneath.
I would try mechanical cleaning with a scalpel or exact-o knife. I would start with stuff somewhere around the
rim. That way if it is the
red crust and
pitting is revealed you can stop and leave the rest of the coin
as is. But if it is hard dirt and you can
pop it off you can continue.
Shawn