If this is in fact Cyanobacteria (red slime algae). There are a few things you can do to try to correct the problem. Two of the biggest contributors to the proliferation of Cyanobacterias are poor husbandry, and inadequate flow. Since you appear to be keeping up on the water changes, and you say your levels are all good, well assume that you have 0 nitrates in the system. Lets take a look at the other areas of reef husbandry that could be a factor.
First and foremost, Phosphate sources must be eliminated. If your phosphate levels are not a problem, then Skip this part. If they are, then you need to identify and cut them off at the source. The most common source of phosphates is the water source, If you use an RO/DI filter for your water, check the output. you may need to replace the cartridges, or the DI Resin. If you Dont have yourown RO/DI filter, I'd recommend you buy one. You can get an inexpensive unit on ebay for around $100.00. Phosphates can also enter the system through many chemical supplementation products. You may want to do a little research on the products you use to supplement your system.
Calcium and alkalinity levels must be monitored and maintained Not only do these two ions feed the corals, but they also feed coralline algae. If water levels are kept in a state optimal for Coralline growth, then the coralline will out-compete the Cyanobacteria for the nutrients.
Magnesium levels will also need to be kept in check to ensure that the calcium in the system remains in a usable state for the organisms that require it. Depending on your method of Calcium and Alkalinity supplementation, you may already be adding Magnesium.
If you are incorporating Macroalgae growth as a means of nutrient export, make sure that your Macroalgae has sufficient light and look into your Iron levels. You may choose to begin dosing Iron to promote healthy Macro growth. Here's a great article on Iron in the aquarium:
http://advancedaquarist.com/issues/aug2002/chem.htm
Now Lets take a look at the flow within the tank. Take a look around and see where detritus is settling in the system. Chances are, it's going to be very near the location where the Cyanobacteria is growing the best. If this is the case, you should take a look at the flow you have in the tank. You may need to increase the flow with additional or more powerful power heads. You may be able to remedy the problem simply by redirecting the flow of your existing power heads. You may even need to look at your rockwork, and possibly do a bit of re-landscaping. You should also take the time every once in a while to blow any settling detritus off of your rockwork with a turkey baster (not used) or spotfeeder.
One more step you can take to help combat is to use your water changes as an opportunity to siphon out this pest bacteria. It is generally very easy to siphon out, and will come out in large mats. If you can arrange to do this just before the lights go out on the tank, you will gain the most benefit as the Cyanobacteria is reported to regress during the hours of darkness, subsequently releasing some of the nutrients back into the water column.
Hope this helps
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