You can treat the soil with a 3% Hydrogen Peroxide (H2o2) mix of 1/2 cup per gallon of water.
You can put the mix in a spray bottle and treat the leaves if it has spread to them.
Algee needs 3 things to survive, water, light and nutes. Remove one of these items and the Algae will die.
1. You can cover the soil with anything that prevents it from getting light.
2. Cut back on the amount of water you give the plants.
3. Kick the 24/7 light schedule while in Veg to 18/6 (In addition to one or both of the previous ways)
You can buy stuff like TerraCyte Fungus Control