A slime monster like The Blob makes what movement? Slither is too snake like. I've thought of words like "gloop" and "drip", but it doesn't convey movement.
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They ooze, in the sense given here:
In fact, recent versions of D&D have classified such monsters as "oozes".
I might also describe such a motion as creeping, assuming that it was moving slowly. If the monster were to be moving more rapidly, than flowing might be appropriate. Glancing at some articles about slime molds, it seems the term streaming is used to describe their peculiar type of motion.
Here's a web page that tells you how snails move, and another that tells you how slugs move. Think of a "slime monster" as a giant snail or slug, both of which are little slime monsters. Because they both create "rhythmic waves of muscular contraction on the underside of [their] foot", you can say that they undulate.
This kind of question can easily be answered with a little bit of searching. First, ask the question: "How do snails move?" Then put the question into a browser's search window. The use a thesaurus to find synonyms for, in this case, wave, and you'll find undulation. Then go find the verb undulate in the the thesaurus