Board Thread:Questions and Answers/@comment-28605250-20160616032724/@comment-27701762-20160616033553

This one has certainly confused a lot of people, mostly because of the "smiling" line. The standard lines are that it is either Flowey or Mettaton, but Mettaton is the only one who truly changes shape. Perhaps more important is that Mettaton reveals later that he hired people to kill you, which fits with Muffet's motivation.

So Mettaton seems like the most likely answer to the question.