Are these good or bad?

I would be willing to bet it was a stomata or how ever you say. It is a beneficial snail that looks like half slug
 
Did it look like a slug with a fingernail stuck to it's back? In that case 1mg is right and it's a Stomatella Snail. One of the best hitchikers you can get! Hope you got 2 so they can start breeding for you.
 
Thanks for the responses guys. I ended up catching it in my fish trap. Turns out there was like 3 of them. After doing a little bit of research i have since released them back to work.
 
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