Alphys lets out a sort of vocalization between a yelp and a bwaaah, and shrinks away even more. Okay, no. None of this is okay. She's never seen Sans move with that kind of speed. Ever.

"T-t-the prince?" Oh god. All the warnings. They had drilled it into everyone that the Prince was dangerous and they had to behave or else. This looks to be a pretty clear case of or else. "U-uhh, wow, gosh, you must've r-really... impressed him if he gave you that!"