Algae need enough of lots of things, including sources of N, P, and various trace elements (and light, space, carbon, etc.).
Having more than enough of one of those will not generally make algae grow faster, because one of the other needs will become growth limiting.
Thus, for example, algae will not generally grow faster with 1 ppm phosphate than 0.2 ppm phosphate because in neither case is availability of P likely to be the thing that is limiting growth.
The Reef Roids and other foods and additives likely supplied enough of what that organism needs, but reducing phosphate will not necessarily control it until the level gets quite low, and sometimes that level is low enough to also harm corals.
FWIW, I cannot tell from the picture what is growing on the sand, but different organisms (cyano, dinos, algae, etc.) have different approaches to helping.