What does the bottle say? If it says its safe for inverts, or doesn't mention that it specifically kills inverts, it should be safe.
The main reason some meds kill inverts is due to copper. Copper hits toxic levels very quickly in inverts as compared to fish or plants.
Many ich treatments use copper and will surely kill shrimp, crabs, crayfish, and most snails.
Former advisor and planted tank geek...life's moved on though.