If you don't have any delicate fish or corals in the tank, you could try red slime remover. That worked for me in my first tank.
If you have delicate stuff already, you could try adding turbo snails. They eat the red slime in one of my tanks.
Then you need to lower your nitrates and phosphates by feeding less, adding a phosphate reactor, or adding macro-algae in a refugium. The red slime, cyano-bacteria, uses nitrates and phosphates to grow.
And use RO water.