So, this guy at my dad's work gave us a saltwater fishtank. It was nice when he gave it to us, but my mom and I don't really know what we're doing. My first question is if anyone knows what this mushroomy thing is and why it's dead. And can it be brought back to life? It's hard to fix our mistakes when we don't know what we're doing wrong, and I'd really like to get our tank healthy so we can add new fish.

Dead

Alive