Spiky Balls?

[BigCrabby]

Hello all! New to the site. Hope this is posting in the right spot.
I'm fairly new to the hobby, probably less than a year. I have a 100g tank and am finding that I probably should have started out a bit smaller. Oh well...Anyways, I've got these spiny looking balls that have appeared in my tank and have no idea what they are. I'm thinking maybe some sort of sponge? Anybody know? Bad? Good?

 

[Joeganja]

Good. Filter feeders @Lionfish Lair

 

[Tahoe61]

Yes to sponge and harmless.

 

[Sm51498]

Sponge! I'm of the opinion that sponges are a sign of a healthy tank.