White growth on maple tree
Thanks for your question. The growth is a mushroom, which is growing out of fungal tissue that is breaking down the dead wood just underneath it. There are several fungi that grow on maples, but I can't see this mushroom clearly enough to be able to differentiate among half a dozen white mushroom structures.
Two things: never eat a mushroom that hasn't been positively identified. And, since the existence of the mushroom is highly indicative of decay in your tree, you might want to consider having a certified arborist take a look at your tree to make sure it doesn't pose a risk to life and limb.
Hope this is helpful.