Damn dude, might want to clean your pieces more often. Personally I wouldn't smoke a bong that hasn't been cleaned in a few days. I would rather stay sober. That being said I have multiple pieces so usually when one is dirty I just grab the next. If they are all dirty then I just roll a joint or use a vape.
1) ew 2) don't smoke that 3) As much as it looks like steel wool, only thing I can think of is like a really mangled resin filled But the more I look at it, the more I think that's just a random clunk of gunk that should be thrown away. Maybe clean your bong again.
Looks like a wad of hair or fur lol. Don't smoke it but I highly doubt it's mold. As for storage, make sure it's properly dried and cured then store in an air tight container.