what is it

[mr.m]

ok anyone know its dark purple and 3 1/2 in long by 2 3/4 wide
thanks mark

 

[spanko]

It is a Scutus. Cool one too. No worries here a great addition to the tank.
Here is a picture of one much like yours form Reef Tank;

 

[mr.m]

cool and what is a Scutus. That pic u posted is what i have mine just poped up one day and no i have not add any thang so it didnt hitchhike in the tank been up for 3 yrs

 

[keri]

http://www.seaslugforum.net/factsheet.cfm?base=scutus
There are links at the bottom of that page on more discussions about these guys as well