These are probably new shoots growing out of the stock that your JM was grafted on to. They can (and should) be removed.

Mutant is correct. Japanese Maples are almost always grafted plants. That's how they clone the various types that have usual features or colors. They take a branch from the special tree and graft it onto a plain 'ole green ("wild") Japanese Maple tree. After the two plants have fused, they cut off any of the wild tree above the graft, leaving just the new branch, and, wal-lah!! You have a clone of the special tree! Look for a grafting scar on the trunk. Anything that grows from BELOW the graft (usually with very different leaves) should be removed, as its the rootstock plant sending up suckers. If you do not remove them, they will eventually over-power the graft.

Regards,

K4