think about what your feeding to your tank and what goes into your tank,your tests will say everythig is good because the algae is consuming the nutrients as fast as they are being introduced, it doesnt take a whole lot of nutrients to keep algae growing.are you using anything at all in your tank to help remove phosphates??? being limited to a small skimmer i would consider running a small phosban reatcor with gfo to keep your phosphates down and then go light on feeding food, rinse frozen food regulare light cycle, but very importantly is keeping your water changes going....consistenct pays
i would work on removing the ditrius youve been noticing either manually or by adding flow to those places
i have halimeda in my sump with cheato i can easily see how its intrusive its got some real grip and clings to the rocks well and would be hard to clear out of a display if its in there...P I T A lol
cheers