While I do agree that the Hepatus Tang needs a larger environment, at that size, I don't think it's contributing to the bioload enough to call it overstocked.
is not easy, or fast, generally. Step one is to measure your nitrates and phosphates and record those values. Step two is to do a massive water change. 50% is preferable. Test and record again. Should see a drastic decrease. Step 3 is to adjust feeding.
Now, I do one large meal a day as well, but it takes several minutes to do so. I feed just a tiny amount at a time. The goal being that all food is eaten before it has a chance to hit the sand. This way, none is wasted to contribute to nutrient overload.
Step 4 is to evaluate flow in the tank. Cyano
cannot adhere strongly to anything, so it tends to accumulate in areas of low flow. Increasing flow will often dislodge it and hinder it's growth.
Sometimes, nothing else works. You can do all of the above and it continues to grow and spread. That is when chemiclean comes in. Chemiclean will make your skimmer go completely bonkers, so it needs to be shut off for 48 hours (can we say nutrient buildup?), and treatment followed with a massive water change before the skimmer can be turned back on.
On the other hand, I've seen far worse cyano
growth than what is in those pictures. If you an keep it at that level, I honestly would ignore it.
One final thought. Though they do not eat it, genereally, nassarius snails help keep it from accumulating by constantly disrupting the top of the sand. Hermit crabs do the same on solid surfaces.