It looks like Gymnopus alkalivirens based on the description, reference photograph, and the fact that this is one of the few mushrooms that fruits this late in the year. This species was formerly Collybia alkalivirens, the Little Brown Collybia.
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- Mushroom Expert: Gymnopus alkalivirens
- Mushroom Observer: Gymnopus alkalivirens- The Audobon Society Field Guide to North American Mushrooms. ISBN: 0-394-51992-2. Little Brown Collybia (Collybia alkalivirens). Plate 108. Page 753.