as nymox said may be bubble algae
don't pop it while removing
i use a piece of pipe on a syphon hose too
or take rock out of tank into a bucket, scrape off then wash before placing in tank
would be good if you gave all the info at the start
if that is the situation may have just been a territorial issue then
but as said, dog may have the taste for it now
i have known a few people in life who spent a long time banging their head on a brick wall
they opened their eyes one day and found a handle in the wall
turned the handle, opened the door
and stepped through to a new world