Grey Mold Algae?

I had that problem with some driftwood. Everywhere I look it just says that it's some sort of bacteria feeding on something that's in the wood (sap or something). Eventually it goes away on its own once the bacteria's food source is used up. You can take the wood out and scrub it and boil it but it will probably come back. Basically all you can do is to wait it out.

Thank you bender!

just regular ol fungus. Leave it alone and it should go away by itself. Some fish/shrimp may snack on it.

I'd increase water circulation in the tank. Like BK said, invertebrates might eat it.

not a problem, throw some trumpet snails at it. Once the wood gets coated in the tank's bacteria, it'll go away.