I have something growing on one of my rocks as well as the back wall. At first I thought it was just an odd form of coralline but I doubt that now. The growths are egg shaped maybe two by three millimeters and although they do not touch or overlap they are found mainly in groups. I have not got a picture of them yet but one more thing I have found is that they feel sort of like the non-slip pads people use in showers. They are starting to move to my back wall, don't seem to be bothering anything I just want to know what they could be.