How old are the T5s? It's possible the spectrum is shifting if the bulbs are old, which could be helping the red slime/green algae to grow.
With regards to your test results showing 0 nitrates and phosphates, this is a common problem in the hobby. A reefer will say that they have nuisance algae, but their nitrates and phosphates are undetectable. This is because the algae in question is using all the available N and P. The rule of thumb is if you have nuisance algae, you have nitrogen and phosphates coming from somewhere. Try a few large water changes in quick succession, and maybe consider changing more water more often, or perhaps changing more when you perform your changes. A good skimmer and carbon dosing (No3Po4-X) will help, but it's possible the phosphates are still high despite those two factors. Generally speaking, carbon dosing will use a lot of the nitrates in the water, but will use a lot less of the free phosphates, meaning more is available for nuisance algae.
One thing to also keep in mind, your sand can over time become full of detritus that will eventually get released back into the tank. Be sure to siphon the sand and stir it occasionally to ensure it stays clean.
**EDIT**
To add on to Dave's comments, "filtration" in a reef tank is difficult to quantify. It means lots of different things to lots of different people. I prefer to think instead about the goal of filtration and how we can accomplish it. The goal is really to keep our tank water safe for our fish and corals. This involves two basic concepts: first, we want to turn the dangerous compounds the fish and living organisms excrete (ammonia) into less dangerous compounds (nitrates). We also want to remove the nutrients that accumulate (nitrates and phosphates) before they cause problems in the tank.
The first objective above is handled almost exclusively by the live rock. The bacteria on and in the rock transform ammonia to nitrite to nitrate. If you have no measurable ammonia and nitrites, your biological filtration (live rock) is working as designed.
The second objective is where all the other filtration methods come in. The filter floss is designed to remove uneaten food and fish waste before it can break down into ammonia and phosphates (note: the filter floss only works if you replace it frequently. It would be best to avoid it all together if you aren't able to replace it every few days). The skimmer is designed to do something similar, it removes food and waste particles before they can break down into nitrates and phosphates. GAC mostly removes toxins and chemicals from the water, and there is some consensus that it can remove other dissolved organics as well. The Nopox is designed to grow bacteria, which eat nitrates and phosphates, and are then removed by the skimmer, thus removing the compounds from your aquarium.
If I were in your shoes I would focus on the nutrients going into the tank and nutrient export. Try to watch the feeding and make sure all the food you put in gets eaten or removed promptly. Also check that your RO/DI water is reading at 0 ppm TDS. On the export side, try to siphon and stir the sand bed, and try a few large water changes.