OK, this is tooo perfect. I took this picture last night.
To the best of my knowledge it's a Neogonodactylus wennerae and a Holothuria impatiens.
The only thing I've seen them bother is hermit crabs. I've got another Mantis about 2" long in a ten gallon tank with several snails, hermits, urchins and small crabs and as long as I feed it every few days it doesn't bother anything. If I don't feed it for 4-5 days it has eaten hermits.
Cold blooded killer, ya, right.