This is called Beard Algae. The cause is introduction of contaminated plants or a bag of fish from a store tank with red algae. The ONLY way to rid yourself of it once you've got it is to either get a Simaese Algae Eater or take all your fish out and clean your tank and ornaments with bleach. I normally would never recommend bleach but this is the only way. You'd have to rinse your tank very well as this is extremely toxic.
The Simaese algae eating barb is the only known fish that eats red algae. The fish is also known as Siamese Flying Fox, and Siamese Fox. Siamese Algae Eater is not very demanding. Suitable temperature is 24-26 C (75-79F). They can tolerate pH from 5.5 to 8.0, but 6.5-7.0 is ideal.
They are not aggressive, they can be kept in any community tank. They should not be kept with Red-tailed Sharks unless the aquarium is large and well planted, because that species is very aggressive towards all its relatives.