What is this?

[smoorman11]

This appeared overnight. Both of the corals nearby look ok. I added a firefish yesterday and haven't seen it since last night. Is it possible for a fish to decompose overnight? Also, The brown spots were blown off the rocks and settled on the bottom.

 

[Privateye]

Hmm. What's the texture like? Is it soft and gelatinous? Are there little specks inside it or is it completely clear?

I'm wondering whether it's some sort of egg sac. Some benthic polychaete worms have blobby egg masses, but usually you can see some tiny specks (eggs) embedded in them.

IMO it's not something you'd want to keep, so if it were me I'd remove it and look for clues. Either in the mass itself, or perhaps a tube in the sand from a benthic invertebrate.