Algae is fed by a couple of things: Food and light. Food is in the form of nitrates and phosphates. Check that to make sure you have a low to no count. The light issue comes down to spectrum. Different algaes will grow better under different light spectrums and light hours. Bright white lights usually produce a green algae. Blue spectrum produces reds, browns and black algaes. The right spectrum and hours of light, in the presence of low to no nitrates or phosphates produces no ( or very little) algae. If your light is an LED
light, you may need to fiddle with the light spectrums produced to clear out the algae. If your lights aren't LEDs, try reducing the amount of hours the lights are on if the algae is green or increasing the light hours if the algae is a darker color. One of those 3 things should solve your issue.