I do not think it is an algae either. I have seen this in new started aquaria and in my sump. Do not know exactly what it is - probably some sort of animals. I think that some hermits will fix it.
Will try to post a longer video later in the night. My tank has been around for about 3 years and recently I discovered such stuff covering the rocks. No idea what are they and yes they do come out easily by either rubbing my fingers on them or by using a brush.