Can't get a definitive ID of that at a macro level. If you want to drop off a sand/water sample with some of that gunk in it, I'm happy to look at it under the microscope for you and ID what you have.
My guess (and I am heavily emphasizing that you should NOT treat based on mine, or anyone else's, advice until you get an ID under the scope): if you test your tank parameters right now, you'll probably have extremely low/undetectable phosphates and/or nitrates, which means it's likely dinos. The type of which, and treatment plan for, will depend upon a definitive ID.