[PaulF]

That very well may be a maple bladder gall. It is caused by a very small mite that ovrerwinters in the bark of the maple tree and migrates to the leaves in May. The gall is made up of leaf material and the mite lives and mates inside the hollow gall. In the fall, the mite goes back into the bark.

There really isn't anything you can do now. Early in the spring an oil spray could kill the mites. While the galls are ugly, they don't pose much of a threat to the health of the tree. The galls will start red, turn greenish and then black. Removing the galls manually will not do much good as that will make holes in the leaves.

That is my opinion, but there may be others who disagree. Plant disease people and bug people may call what you have the glorious wonder of nature at work. I would rather gas the buggers and get back to nice green leaves next year.