If you're going out of your way looking for a monster that terrorizes people by melting in front of them, I guess this is the movie you'll want to watch.
The Incredible Melting Man. I don't know how incredible he is, but the rest of the title fits.
Stop the world and melt with my review. (http://thegreatwhitedope.blogspot.com/2011/02/incredible-melting-man-1977.html)