It’s straight up mushroom. My limited understanding is that you can make mushroom paper from a number of species but Trametes versicolor(turkey tail) tends to work best. Mushrooms with pores under the caps, instead of gills, are preferable. The cellulose and chitin in Trametes acts as the binding agent-no base necessary. You can mix Trametes with other varieties to experiment with colors and textures. 

We broke down dried mushrooms to make the pulp. It doesn’t matter if it’s dry or fresh, dry just takes a bit longer but also preserves what you don’t have time to work with. A home blender is enough to get the job done. Just blend till you get a mushroom smoothie and then slowly add cupfuls to your water vat. Now you can shape and dry sheets as you normally would.

Mushrooms for Dyes, Paper, Pigments & Myco-Stix by Miriam C. Rice is a good resource. Currently out of print but possibly on a mycology class bookshelf?

Good luck on your project!