Tremella mesenterica parasitizes Peniophora. It has often been reported as parasitizes Stereum, but it turns out that is T. aurantia.
Based on Peter Roberts article (see below), “microscopically, Tremella aurantia can be distinguished from T. mesenterica by its smaller more subglobose spores, smaller basidia, and the presence of unclamped host hyphae in the subhymenium and context. The hymenimum is normally covered by a thick muscous layer full of basidiospores and yeast-like conidia, which presumably is why the surface of the fruit body appears matt and often pruinose.” He also notes that macroscopically in general T. aurantia is more matte and T. mesenterica is more glossy.
Here’s the spore size info from the paper: T. aurantia (Q = 1.1-1.4), 5.5-9.0 × 4.5-7.0 µm. T. mesenterica (Q = 1.2-1.8), (8.0-) 10.0-16.0 (-18.0) x 6.0-9.5 (-12.0) µm.
This distinction was brought to my attention by Else Vellinga who provided the follow references.
Peter Roberts from Kew Gardens has written several articles on Tremella in Britain:
1995: British Tremella species I. Tremella aurantia and T. mesenterica. Mycologist 9: 110-114.
1999. British Tremella species II. Tremella encephala, T. steidleri, and T. foliacea. Mycologist 13: 127-131.
2001. British Tremella species III. Tremella callunicola sp. nov., T. invasa, T. sarniensis sp. nov., T. simplex, & T. versicolor. Mycologist 15: 146-150.
2007. British Tremella species IV. Tremella obscura, T. penetrans, T. giraffa & T. polyporina. Field Mycology 8: 127-133.
the first one treats the species we see: Tremella mesenterica parasitizes Peniophora species, T. aurantia is growing on Stereum.