Apparently, Dream showed up in Uki's chat earlier.
Dream, being the attention whore he is, wanted Uki to acknowledge him, but Uki was too engrossed in his Minecraft scavenger hunt to care. He instead got recognized by Fuuchan, who hit him with the "who?" line.
edit: I guess I should say that I don't know if it's actually him. I just thought it was funny.