Amphidinium
The dino bloom in my aquarium are largely Amphidinium sp. This genus of dino are often mis-identified as diatoms, because, to the naked eyes, they are almost identical.
The macroscopic appearance of amphidinium bloom is a brown dusty appearance on the sand bed. There is very little stringy mucus. Unlike diatoms though, amphidinium generally does not encroach onto the rock work.
During night migration, unlike many dinos which migrate into water column, amphidinium migrates deeper into the sandbed instead.
Microscopically, amphidinium does not have an armored shell (theca). It can be identified by the asymmetric cleft, which looks like a beak.
It has two flagella (whips) which helps it to move rather effectively in water. It has a nucleus, and it is capable of both asexual and sexual reproduction; can even form inactive cysts when the environment is less than ideal.
If we consider the characteristic of amphidinium, there is no doubt that it's a resilient organism, difficult to control in an aquarium setting. Since it does not migrate into water colomn, UV is ineffective against it. Due to its fast reproductive rate, it can easily overwhlem other macro and microalgae. The only "good" thing about it is that it has very low toxicity. Therefore the clean up crews does not perish during amphidinium outbreak.