We have a very old Maple tree on our property. I noticed today that the leaves are covered with little green things...looks like they could be eggs of some sort
Anyone know what it is? Will it be harmfull to my tree?

It's probably a gall from a mite or some insect. Young critters growing inside. It's also probably been getting these for many years. There are none on neighbouring trees because these types of critters are usually host specific, or genus specific at least. I wouldn't worry about it. It shouldn't harm the tree. Just part of natures' interdependence.

Yep They are leaf galls. Very common to maples. The insect that is usually responsible for them is a parasitic wasp that is actually beneficial to controlling other insects pests. While it may look bad it does not bother the tree what so ever, and having that population of beneficial critters around should keep your other pests in check.

Thanks Longy and Plant Doc !