It means stimulation. Like for people with Sensory disorders it stimulates them and soothes them just by playing with and feeling the texture of the slime. It doesn't mean any thing dirty don't worry.

Here's a better definition courtesy of Bing

Stimming
Self-stimulatory behavior, also known as stimming and self-stimulation, is the repetition of physical movements, sounds, or repetitive movement of objects common in individuals with developmental disabilities, but most prevalent in people with autistic spectrum disorders. It is considered a way in which autistic people calm and stimulate themselves.