Galls, Diseases, and Pests - Filbertworm Moth
Cydia latiferreana
The Filbertworm Moth does not cause a gall to form in an acorn. Instead, it lays its eggs so that the larva (caterpillar) can eat the acorn as it grows. So the Filbertworm Moth is a pest. There were two live larvae inside this acorn.
You can see where the Filbertworm Moth larva has eaten part of the acorn.