What is this?
 

It started off with just a few but now I'm finding a lot of these little guys so they're definitely reproducing. Does anyone know what they are and if they're good/bad?

I'm attaching pics but these little snail looking things have 2 antennas, they have a long slug like body and the mouth is exactly like the turbo's I have in the tank. The small ones are probably 2mm x 4mm whereas the bigger guys are reaching 4mm by 10mm. They're mostly nocturnal.

I've never had a tank with these so I'm wondering if anyone else knows what they are?

Stomatella snail

I believe it is in the same family as the nudibranch. I have one in my reef system, as well. Mine is not multiplying, has not done any damage to the system, and only comes out when the lights are out. All it does is graze on algae.

Same class as nudibranchs...both gastropods.

Stomatella varia. It's a herbivore and it's harmless. Great little algae eater.

+1 I have a few in my tank I thought they were nudibranches at first, but these guys are beneficial where as nudis are detrimental in most reefs depending on the variety! keep him like said above he's a good little algae eater!

Great! Relief to know that it's just a grazer. Thanks for the quick help!

For future reference to IDing the wonderful critters that show up in your tank

http://www.chucksaddiction.com/hitchhikers.html

Like the others said it's a stomatella. I had a couple when I transferred my tank a couple months ago as most of them died off cause I left my LR out of water for to long. In a few months I now have hundreds and they are breeding like crazy and great algae eaters forsure.

I have some of these guys and they just popped up out on no where , hadent added anything to the tank at all for almost 4 months.they do a great job to bad most places dont actualy sell them.

yeah, but they reproduce pretty quickly, mine spawned a few weeks ago I wasn't sure at first but yup thats what it was, theres smaller ones in there now too!