I was reviewing the Dino Treatment article and the Are You Tired? thread by @mcarroll , and I noticed that many of the pics people were sharing did not have quite the mat structure I am seeing on my rocks.
Right now I am waiting to see if anyone in my local group has a microscope and can help me identify, but in the short term I am just following some of the advice in those two threads.
Based on my tests (PO4 - 0.02, NO3 - 0-5) I am almost 100% sure it's dinos, but other people's pictures have me 2nd guessing.