Unknown rock growth, spring maybe?

[asnyder]

Sponge

 

[asnyder]

I’m about to sanitize some old dry rock and start curing it for a new tank build a couple months out. This crumbly red stuff on this one rock I have looks to me like sponge, but I wanted someone’s more experienced opinion. Please let me know if you have any idea. Also going to re-removing as much as I can from this rock if anyone thinks it is sponge, I’m probably not gonna use it since sponge can wreck havoc on a tank.