Quick update - I do think this was a form of algae. I was able to knock off some of this during the weekly water change but some "held on" with the vacuum and even a light rub of the finger. Given the tank gets a vacuum every week I wouldn't think it was just particulate matter. Anyhow, I have since added some Amano shrimp and they seem to be chowing down on whatever it is. More flow in the tank seems to be helping as well. Thanks for the input!