Yep, some kind of fungus. No specific idea but something that likes newspaper. Almost certainly harmless or beneficial for your pile (don't inhale it, though).

Often dry my castings this way but have never seen this before…… interesting.🤔

PS have a good New Year

Worm castings with newspaper around and just enough moisture and air is a perfect environment for some random cellulose-loving fungi to grow quickly and then almost as quickly disappear.