White vein/veiny strings

[thisguy1]

Hey I was hoping someone can help me. I recently seen these white veiny type of strings growing on my glass and frag rack. I’ve scraped some off days ago but it’s growing back. I’ve never seen this before and I’m hoping someone know what this is

 

[Topekoms]

Sponge they are harmless

 

[Justin Cook]

They do look like some sponges I've had experiences with. If you pinch them off with your fingers, so they sort of disintegrate? The one's I've had were larger in diameter and not quite so stringy so I'm not convinced they're the same things.