What is this?

[monza]

Personally I'd take the rock out and dig /smash around the aptaisia area completely removing it and any signs of it from the rock. Actually though, if it was me I would not even use the rock, it's a plague that may take years to get a foot hold but if it ever does you'll under stand. I see a few in your picture which means they can and probable are hiding every where in the rock.