It sounds like cyanobacteria aka blue green algae and is not an algae but a bacteria.
It tends to do well in high phosphate conditions and often in low flow areas.
What food do you feed, and how much?
Are you keeping goldfish?
How often are you changing water, and how much do you change? Do you clean the filter at the same time and how do you clean it?
Do you have many plants?
cyano is in nearly all tanks, just some times it will run rampent...