I could be wrong but those look like dinos to me.
http://en.m.wikipedia.org/wiki/Dinoflagellate. If thats what it is, I just finished fighting this stuff and its not fun. But my advice would be to syphon the hell outta the sand and do 80% WC for 3 days in a row. After that wait a day or two to see if you have beat it. Repeat if you still have signs of it. Thats what i did and it was gone in 4 days. Not to mention my tank has never looked better since i did. Just my opinion