In planted tanks it's not uncommon to find algae on the back glass, driftwood, and larger rocks. A small amount of algae is perfectly normal, and it is nearly impossible to eliminate all algae. The goal is to get rid of nuisance algae, not all algae. Nuisance algae spreads extremely quickly and detracts from the appearance of the tank. There are actually some algaes which are normally considered a nuisance algae in the US but are cultivated as being highly desirable in other countries.
If what you are looking at on the driftwood is no more than say an 1/8" thick and just makes the the surface of the driftwood fuzzy and green, this could very well be algae. I've got some growing on a piece of driftwood and I think that it is very attractive and helps the driftwood to look more like a part of it's surroundings. That tank is by no means suffering from algae run out of control.
On the other hand you could also be looking at some sort of moss as many members have suggested. Rather than being a thin, even coating across the surface of the driftwood it looks like a bunch fuzzy of branching strands growing out from the driftwood. Check out some of the links that others have given to Tropica and Plantgeek for some excellant pics of what these mosses look like.
Java Moss is low light tolerant, but like all plants it does have some minimum requirements. Just how low light is your tank?