[Sk8r]

Cope, that green could be algae, but maybe a local variant: if they operate like nassarius, great. I've never seen green; I've never actually seen brown, except in some odd internet offerings, and I do know that there is an Ebay seller who is collecting odd snails out of the Caribbean and selling them. I am really not keen on Ebay sellers of livestock: you don't know what you're getting into if you don't know the hobby well, and if you do know the hobby well, you know other reasons not to deal that way, including the chances of getting illegally collected items, disease, you name it. I have actually seen an Ebay offering of 'anemone' rock. It was, of course, covered in aiptasia. Clever, but not real nice. And I'm betting some novice marine keeper bought it.

On the brown ones, watch their behaviors closely: be sure they're not skinny-shelled on one long end [ever seen a whelk shell?]. Nassarius are fat-shelled more or less all over, have a distinctive forked tail [they look almost like a flatworm with a shell!] and can't climb glass or rock---they're SO slimey they slide off or U-turn. One thing all true nassarius are is---fast. Only a stomatella [good guy] snail can keep up with them.

Whelks are something I probably should mention: they are as described above, and they envelop and kill, crawling over their victim. Fish don't stand still for this, obviously, but a whelk will be after anything it can get.

Some people are scared of fighting conches...the name is a joke. THey're gentle creatures who 'buck' to dislodge irreverent crabs and other snails that might climb onto them. The only thing you should NOT have in the tank with a conch is a plate coral: conches have a jagged spur they use for pole-vaulting over small obstacles and for defense; but it rips heck out of plate coral and will kill it by accident.