You can remove the plants and give them a soak in Hydrogen peroxide 3%. I have had the same problem several times. I usually get a 1 gallon container of tank water and add 1/2 cup of hydrogen peroxide and soak the plants for about 15 minutes. If you have a soft bristle toothbrush, you can gently scrub off the algae after the 15 minute soak. Do a second rinse or the plants in tank water to remove any traces of the hydrogen peroxide. Hydrogen peroxide breaks down under light, so run your lights for about 2 hours after replanting. At this point, you can replant the plants (also a good time to use root tabs if you have plants that are root feeders). This is also a good time to do a good vacuuming of your substrate. Finally, if you keep getting this problem, reduce your photo period to 6 hours per day and see if this solves the problem. You can always work your way back up to longer photo periods if the plants show signs of yellowing, just make sure it is not a micro / macro fert problem. Remember that root feeders like iron, so keep an eye on this.