Something to ID

Water for water changes from the LFS is not typically taken from their tanks, it's taken from an RO/DI system then has salt added, just like we do at home. They use it in their tanks, not take it from their tanks.

The pink & green algae in the sand looks like cyanobacteria to me (I have it in my refugium, tank up since October).

Not sure what you're asking about in the 2nd pic.

Dead coral can have a different color skeleton, so that's probably the color you're seeing in the dead coral but it's hard to tell from that pic.