what is this?!?!

[edress714]

It's only on this rock (of course I got it from Petco) 

[surf&turf]

Are you referring to the white stuff?

[ParrotFish]

Looks like baby sponges

[surf&turf]

Looks like baby sponges

 

+1

[edress714]

Are they bad for the tank? (No clue what they are)

[ParrotFish]

If they are sponges, no not at all. Sponges are filter feeders. Some times they can grow places you don't want them (like on zoas) but other wise harmless.

[DuffyGeos]

Are they bad for the tank? (No clue what they are)

No they are good...if they live. Some fish will also eat them.

[edress714]

Ok thanks, well my zoa is on this rock lol

[YHSublime]

You shouldn't worry yet. I have sponges that I don't care for growing on my frag racks. Unfortunately, I think the fish that will eat them are the same fish that would enjoy my zoas just as much!

 

Don't stress it, unless they look like they are getting out of control.

[davelin315]

If they are hard, could also be a foraminiferen.  Looks more like a sponge, though.

[SurlyT]

I agree that sponges are harmless in terms of affecting other animals, but some of them can be surprisingly invasive. I had a type of black finger sponge that grew everywhere in my tank and was tough to keep under control. But yours looks nothing like that, so I see no reason to worry about it.

[Origami]

Some sponges can be invasive. (Some that I've seen were white/gray that would spread with little finger-like projections.)  I'd just watch it and see if it becomes a problem. Looks pretty benign right now.

[thakki]

I recently started seeing them in my tank too. They seem like spreading faster...will it be okay to brush them off?

[Origami]

I recently started seeing them in my tank too. They seem like spreading faster...will it be okay to brush them off?

If you're trying to remove them, then remove as much as you can first from the tank. That's probably the best way. Some can be easy to remove using a pair of tweezers first, and then (as you say) brushing off the residual.