This is what I've done: dip the plant into boiling water for a minute or two, when it cools rub the algae off with my fingers under warm running water. DH thinks this is pretty gross but it works really well, especially for the brown algae.

Now I have mostly live plants and only one silk plant, I allow the algae to grow on it because the plant has red and green leaves, the algae is dark green and pretty furry, it makes the plant look a little less fake.

If it is the brown algae (diatoms) and this is a new (1 month to a year) aquarium wait until the brown algae disappears. It should when the silicates leach out of the new aquarium glass, silicone seams and plastic equiptment. More frequent water changes and light increase will help it along a little. When the algae goes away put your plants back in. Constant washing them will fade them quickly. If it is green algae then reduce lighting hours, a blackout of a week and frequent water changes will help. If this is a persistant algae test water for nitrates and phosphates, you may have alot of nutrients in your water, sometimes tap water is allready high in these nutrients and disolved organics boost them.

Try soaking them in chlorine (1 part to 10) for about 10 minutes. The fabric plants are made of man-made materials, and are more colourfast than natural fabrics. A ten-minute dip, rinse well, allow to dry in the sun. The chlorine will kill the algae.