A little background info. I have an exoterra 18 x 18 x 24. 1/2 ground cover 1/2 water. I have a Terra Lite base substrate for the land which rises 1 inch above the waterline. Gravel put down in the water area, a reptiflo 200 pump in the water. Nothing circulating the air such as a fan and so on. The moss covers 90% of the ground area leading up to the ghostwood, which acts as a border for the shoreline.

Sounds like algae or possibly early stages of moss, they look similar. Even actual molds and fungi on wood (white, gray, other colors than green) are normal in a new tarrarium though- sometimes little mushrooms will pop up. It's a usual part of getting everything in balance. It almost always all clears up on its own, after a few weeks to a couple months.

I'm glad to hear that! I appear to be starting mushroom farming myself, and am not especially happy about it. One tank is new (made late May), but the other one is one year old.
-Jmcat