It is a nuisance algae, if you can take the plug out and remove the piece of algae go ahead. Then just ro/di dip it, and put it back in. If you can't remove it just carefully manually remove it in the tank.
You have nitrates even if it is a low amount, that's why it grew, but it isn't a outbreak to freak out about. Keep your nitrates low and it won't have a chance to grow.
There looks like some green algae on the rock, you may have some elevated nitrates. How are you keeping up with exporting nitrates? Water changes? Skimming?