There's a slug in my pill bug habitat. Is it a danger to my isopods or can I leave it in there?

Unlikely. Not that pillbugs are quick, but slugs are much slower. And I don't think they are equipped to predate on such large prey.

Due to the very different physiologies between the slug (mollusk) and isopod (arthropod), it is unlikely that a parasite carried by the slug would harm the pillbugs.

But why do you have a pillbug habitat? Just a bug-lover? Interesting.

If it came "from the wild" it could potentially carry mites or nematode parasites. That would be the only way it would threaten your entire pill bug population.

(There are definitely nematodes which can be carried by slugs without killing the slug, which can infect and kill pill bugs or pill bug eggs, for example Steinernema carpocapsae.)

It can be dangerous, as slugs are carnivorous.

They could also be helpful, as they are also decomposers.

But expecct a slight decrease n your pillbug population.