Based on what I can find here and elsewhere, I think Richard's the grand prize winner. Sure looks like the photos and descriptions of dinoflagellates I can find.

Sounds like this stuff is a HUGE pain to get rid of. Here's my tentative plan:
1. Continue manual removal
2. Turn lights (2x250W MH) off for 24 hours, then turn back on with a reduced cycle
2. Start thawing food (home-mixed and frozen) in R/O to soak off phosphates
3. Boost alk and pH (pH is 7.8 or so during the day, which is probably part of the problem; alk is about 14 dKH, Ca is off the chart). This is tougher to do, as I haven't got room for a kalk reactor or even a top-off bucket anywhere behind/beneath the tank. Looks like it'll be manual dosing for now.

Any other suggestions?

Chris