I was going to enter this photo in a contest, but the rules for entry were so bothersome I decided against it.
Please explain what it is.
This is a sapsucking slug or Costasiella sp. they are approximately 7 mm in size. Photo taken in Puerto Galera, Philippines.
I do not mean to sound glib, but this is an Underwater Photography forum. As a suggestion, get a good book on reef creatures; I suggest one by Paul Human and Ned Deloach.
If you want to reply, then
register here. Registration is free and your account is created instantly, so you can post right away.