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If you take it off with a toothbrush, it will just go on somewhere else, also I had the same thing in one of my tanks a while back, and it just kept coming back so I turkey bastered it, and then got some siamese algae eaters and it was goneIt's a 40 gallon tank that we restarted about a month ago. We did a tear down to control black beard algae. There's approximately 25 fish.
The material on the rock came off easily with a toothbrush, so yes, I tend to agree it's probably not algae.