Help identifying white crystal/webbing on rocks

[spaghettinick]

This white stuff has been growing in the underside of my rocks and behind some corals. Thought it was just some sort of sponge, but looking for confirmation.

 

[Tahoe61]

Sponge.

 

[spaghettinick]

Sponge.

Do you think it’s safe to just let it keep growing? I thought sponge fed on silicates but the tds on my ro/di is reading zero.