greyish green huh? TBH its hard to say without pictures. Are you lighting the rock while it cures?
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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.