I am not sure but I think the stuff on sand is GHA which is main thing I’ve been battling for a while. Doesn’t look like images of dinoflagellates I see online. Here is a closer picture if you can give more feedback :)It looks like you may have dinoflagellates, zero nitrates and the brown patches on the sand bed seems consistent with it. An option would be to dose calcium nitrate, potassium nitrate or sodium nitrate to elevate your inorganic nutrients.
not sure if that’s the cause for your corals not doing well although it may well be connected







