From what i observed, this mod only works for elevator levels. So you can just enter a random level and come back until it spawns on an elevator level and then go there.
I can confirm that this mod no longer works. People who say that this mod still works have just been lucky: they found the slim and thought it was thanks to the mod, when in fact it was just chance.
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it worked great before though too, you just have to pause time for the slime to remain on the floor it showed it was in
Arbitrarily shows you a level where a slime can spawn, and that slime MIGHT be prismatic.