Is it safe to eat leftover mushrooms?

Yes. However, raw leftovers must be stored at temperature of 34°F to 40°F. To prevent moisture and bacterial growth, store them in a paper bag. Conservatively, leftover mushrooms should be consumed within two days of purchase. Mushrooms may keep longer but be sure to look for signs of spoilage. Freezing raw mushrooms generally isn't recommended.
 
Cooked mushrooms should be stored under the same conditions and consumed within three days. Alternatively, cooked mushrooms can be frozen for up to eight months.
 
Source(s):
GourmetSleuth.com "Food Storage Guidelines"
Canadian Food Inspection Agency "Packaging and storage of fresh mushrooms"
Health Canada "Safe Handling of Mushrooms"