I found this on the Internet.
What Makes Red Slime Algae Grow?
 Nutrients that come in the form of nitrates, which are produced from DOCs (Dissolved Organic Compounds).
 DOCs are produced from excess broken down organics, such as from detritus, uneaten foods, and decomposing plant and animal matter.
 Phosphates and silicates are other nutrient sources to take into consideration.
 Providing too much or the wrong spectrum of light. It seems that red slime algae grows best in the 555-564 nm (manometer) wavelength.
Solutions?
We suggest that you don't try to put all of these solutions into action at one time! Start with one solution and see what results you get, and if that one doesn't work, try another one, and so on, until the problem is resolved.
 Adjust or change the type and/or duration of light being provided.
 Red slime algae prefers the low water flow areas in an aquarium. These low flow or calmer water areas are prime spots for it to get started and take hold. Improve aquarium water circulation by adding a powerhead or two, depending on the size of the aquarium, or a wavemaker or surge device.
 Eliminate the algae's food supply by controlling DOCs, nitrates, phosphates and/or silicates. The most effective way to get rid of excess nutrients is to reduce the cause of them. This can be accomplished by cutting back on feedings, siphon up detritus and uneaten foods off the bottom of the tank regularly, and by adding a protein skimmer.
 You can try using an additive to "cure" the problem, such as with Aquarium Instruments Ultralife Red Algae Remover as one example. However, many of these types of treatments appear to only solve the "symptom" (red slime algae) of an underlying problem. Cyanobacteria are a form of bacteria, and many of the additives currently in use are antibiotics, which are medications that can weaken or totally wipe out the biological filter base of an aquarium. Use these types of treatments cautiously!
 Luckily, this algae does not attach well and can be siphoned out of the aquarium to help prevent the spread of this nuisance, while you determine and correct the cause of the problem. If all else fails, you can harvest your red algae and sell it. Certain varieties of red algae have been shown to be effective in treating the symptoms of herpes and also to be a valuable food supplement. However, we wouldn't recommend just eating it right out of your tank, because some varieties of red algae can be very toxic.
Red slime algae is not really all that hard to get rid of. If you determine the actual cause underlying the problem and take of it, your tank will be much healthier, and you can eliminate a great number of challenges in the future.