Duanes is Probably correct about what the issue is. But I'd have to see a good picture of the algae to be 100% for sure.
If its cyanobacteria, also known as blue green algae or slime algae. Then there is only one way to kill it. You would have to do a good water change about 80%. Be sure to try and get most of it physically removed. Is it on the substrate? If so vacuum very well. Then after doing the water change, you will need to do a full term dosage for a week of erythromycin. During this week you also need to have a 100% lights out/off. No light of any kind for the entire week can be allowed. It has to be pitch black dark for the week. Use dark towels to cover the tank fully and keep the lights off. At the end of the week 7 days. Do another large water change. If there are still signs of it then repeat.
Causes of this issue is from several things. Is this a live planted tank? In live planted tanks a lack of nitrates with light bulbs that have gotten old and low current flow throughout the tank cause these issues. In a non planted tank poor in tank circulation, use of improper lighting and high organic load is a major contributor. The later being the biggest issue due to Overfeeding, or excess organic matter in the tank can trigger BGA.
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