Thanks Jim and Pete! I did a web search using your info and found this on Wet Web Media along with a picture for positive ID:

"The "brown wafer algae" Lobophora is fast growing but not at all palatable to most herbivores. Some Diadema urchins will eat it, and if your tank is over 100 gallons, then a Naso tang may control it for you. Else, manual extraction is called for (Ughhh!... no fun)."

I checked my phosphates and they were 0, and I have Chaeto growning my my sump. I did move the tank a few months ago, though. Guess I have to keep picking at it.