Algae? Or Dino or just build up

That's a tough question. There are something like 2000 species of dinos. They range from the symbiotic algae in our corals, to the stuff you posted a picture of, to a type that causes a very fatal fish disease called marine velvet.

Generally they are a lot like nuisance algae and feed off of excess nutrients. However, typically nuisance algae will out compete them for nutrients and they don't gain a foothold. Some however, can even consume organic carbon, sometimes even in the form of other organisms to gain required nutrients and can survive in very low nutrient environments. Often they gain a foothold in that situation, as they are able to out compete algae in such an environment. In some cases, the decaying algae, dying from lack of nutrients may fuel their growth further. Others may form complex food webs with other organisms such as cyano. I think it is common for dinos to show up in association with cyano, likely for this reason.

However, a lot is unknown. Varying chemical compositions tend to fuel different sets of algae and other nuisance organisms, based on which ones are best able to compete in that environment. Generally keeping up with maintenance and not slacking on husbandry is a good first step for prevention. It's also important to realize some of the most important parameters, are those we can't test for such as levels of organic compounds. These are removed primarily by GAC, as well as protein skimming. Water changes can also remove them, although, water changes replenish trace elements, which in some cases can fuel them (and nuisance algae BTW).

In addition to Corailline's link, here is another good read.

Dinoflagellates - Predators, Pathogens, and Partners by Eric Borneman - Reefkeeping.com