Where abouts is the new growth, on the tree? If right at the base then it sounds as if you have a grafted tree and the rooted part has grown. In that case you need to remove it as close to the point of origin as possible.
A picture would help.

If possible rub it off rather than cutting off... Oh and did you mean Acer? Never heard of an 'aver'

We had a hawthorn growing from an ash - it seeded into a hollow and took root there! Don't expect it will last more than a few years.

I've had this happen too! I did what moongrower says! I had a green one growing on the stem of a red one! :)