Hypomyces chrysospermus

Common Name: None.
Description: Fruit body a 3-stage mold that grows on boletes. At first it is white and moldy, then it becomes yellow and powdery, and finally it becomes reddish brown and pimpled.
Habitat: singly or in groups on boletes; also reported on paxillus and rhizopogon mushrooms. Common. Found widely distributed in North America.
Edible: No, possibly poisonous.

Source: Mushrooms of North America by Roger Phillips.