The tiles you have used for the shower and floor would work if you have any left. Or find a mosaic tile that has one of the colors in those tiles. I would stick to something fairly plain - a stone or faux stone, perhaps. Maybe you could go for an interesting shape of tile, in a simple color. I would not go for one of those matchstick mosaics in glass and stone for instance. Too busy for a bathroom where you already have a patterned stone tile. I am not a pro, this is just an opinion of a random stranger!

Yes, if you really want a backsplash, I'd take the smaller mosaic from your shower. You don't want to add another set of colors or patterns.

Thank you! I did not have any tile because we moved in with this bathroom being as is. I don't necessarily want the back splash but someone suggested its a good idea... in order to protect the wall? Thoughts?

I would not do a back splash. Just hang a mirror and be done.