What on earth is this!? Slug? Snail??

[Jordan Pascuzzi]

 

[bonghapster]

Hard to tell from the photo.

 

[Nick Steele]

Possibly a wedged sea hare. Tough pic to see though

 

[Nick428]

Looks like an upside down jelly fish
Or maybe a tunicate.

 

[Reefer Reboot]

Google Cowrie snail

 

[BranchingHammer]

I'm thinking cowrie as well with its mantle extended over its shell.

 

[Tired]

Looks like a cowrie. Poke it- does the soft part retract and reveal a shiny shell underneath? If so, take a picture of the shell in white light for a better ID.

 

[KenRexford]

It’s definitely a money cowrie, IMO. Looks identical to mine.

 

[vetteguy53081]

Cowrie, but pic too blue to determine what type . They are peaceful, and should not be kept with more aggressive species that can possibly harm them. For this reason they do best in mature tankswith well established live rock. Algae is their food of choice