I found this in my search
They are among the hardest marine tank nuisance algae to properly identify. They range in color from: dark brown, to light green, to colorless. Most of the time they look like a stringy “snot” and will sometimes have what looks like to be bubbles on their surface. Dinoflangellates are also toxic to most tank inhabitants. Finding a large number of dead snails in a short period of time can be a sign that you have dinaoflangellates in your tank. Due to the toxins, more frequent water changes, manual cleaning, and lots of carbon are recommended in addition to the described control measures in the below sections. Dinaoflangellates can grown on any surface of in your tank.
The below picture is the Dinoflangellates a friend of mine had in her tank. It’s green-ish in color. The picture is not the best quality, but this should give you a idea of what to watch for.