The sesame seed shaped dinos are ostreopsis.I pulled some sludge off of one of my snails this evening and put it under a microscope. It appears to me to be a mix of the previously demonstrated dino (probably prorocentrum?) and another type that seems to spin around in circles with more of a teardrop appearance. I probably shoudn't be surprised that there are multiple types of dinos as the conditions have been right for them, but am bummed nonetheless. ID would be helpful if people have opinions.
I'm continuing to dose phosphate and nitrate. Phos back down to 0, so will be dosing twice a day starting tomorrow to try to get levels higher for biodiversity. While it hasn't yet taken over the tank, I'm concerned that I've caught it too late to turn it around before it does...
I believe you are running a UV, what size and how is it plumbed?