The term slippery slope has legitimacy as a precaution, where assuming a moderate position may lead through fits and starts to taking an extreme form of that position. The fallacy is in invalidating that moderate position simply because there is an extreme version of it. “I support a minimum wage.” “Then you must be a Communist.” “We can’t legalize marijuana, because then we’d have to legalize everything.” “I’m planning on taking up running.” “But I heard that marathon runners have a higher incidence of having heart attacks.”
Remember, the fallacy is discrediting a position because there exists an extreme of that belief, but there is legitimacy in expressing caution over an ideological drift over time. For example, perhaps it’s better to to combat all forms of government censorship rather than allow bits and pieces to accrue over time due to creeping normalcy and risk truly curtailing freedom of speech.
Note that this fallacy is also sometimes called the Camel’s Nose, apparently.