scrub it all off then see what happens, if it keeps coming back you may have to treat it but i am not 100% sure if it is harmful or not (although i am not 100% sure it is a fungus), better asking in the emrgency section aswell.
It comes off quite easy by rubbing it with your fingers, it feels very slimy. Local fish shop said that it is algae with out the chlorophyll. Unsure what I should do, only have had the tank 3 weeks