It looks like it could just be a person that didn't cut the bud eye in properly. If you don't put it in correctly then part of it can slip or push out when it grows.

That does look strange to me. I would email a photo to the seller and ask them what they think, besides putting a few more naked pictures on here so we can see the roots too. I'm having a little trouble, without seeing the whole plant, knowing what that is.

It does look like the beginning stages of Crown Gall to me. That white growth eventually darkens and look more woody.

What company sold you this rosebush?