It's probably athenia or what ever it's called in most cases it can be pulled off in one piece that is if you can get the edge of the mat to start to lift. I have some that I trim pretty much regularly pull rock out. I then use the edge of my finger nail and kind of flip the edge up and then it pulls off in fairly large pieces. you may want to applie a bit of boiling water to the spot useing a turky baster and have the spot you are basting pointing down ward so the water runs off and only afects that area this will kill the remaining athenia. The stuff you pull of you can then sell or give away.

Bill