Is it growing in round spots (coralline) or in sheets (cyano).
If you touch it with your finger is it hard (coralline) or slimy (cyano).
If it turns out to be coralline this is desirable (except not on glass you wish to view through) just scrape it off with an algae scraper or credit card.
If it is cyanobacteria / blue-green algae thsi is not desirable. Usually shows up when tank is cyclign (i.e. in new set-ups) and is usually associated with high levels of dissolved organci carbon (DOC).
There are a few things u can try to get rid of it.
1) Manually siphon it out
2) Run activated carbon to remove DOC's
3) perform regular water changes (10%/week)
4) Increase aeration. This increases the dissolved oxygen concentration which helps to oxidise DOC's.