Very pretty! It's the fruiting body (mushroom, sort of) of a fungal organism. Is it growing on a fallen branch/log or a living plant? If it's on one that is dead, it's a decomposing fungus. If it's a living tree, then it's a sign of a disease column within the plant.

I've not seen this one in person. I'll get out my Diseases of Trees book and see if I can find it.

It sort of looks like this Cauliflower Mushroom, what do you think?

Here is a link that might be useful: {{gwi:834225}}

The branch is dead and fell this spring. It's just waiting for October so I can burn it. I have never seen this particular fungus before and it is actually quite striking and brilliant snow white.

they do say beauty is in the eye of the beholder.....

;^)
rb