greyish green huh? TBH its hard to say without pictures. Are you lighting the rock while it cures?
Did it come off when you moved the LR, or does it move now you replaced your powerhead? Ive seen a similar sponge, but that holds tighter to LR, and wont fly off if you took it out and washed the LR. If this stuff comes loose easily, id suggest getting a bucket, half filling it with tank water, and rinsing the LR to remove it.Then re-fill with new salt water.