White spot on hammer

[Lostreef]

Woke up this morning and noticed this white spot on my hammer before the lights started coming on. When the lights came on, I noticed it retracting into the hammer. Any ideas?

 

[JGT]

Hard to see, looks like sponge or tunicate.

 

[KJ]

Woke up this morning and noticed this white spot on my hammer before the lights started coming on. When the lights came on, I noticed it retracting into the hammer. Any ideas?

Sponge.