(ok, maybe 250 is the absolute lowest with chance of a surviver, and that is incredibly unlikely) That is probably adult size, (haptosquillid maybe) or just very young. You can throw it back in your tank. It will do very little harm. I doubt that it could do more than kill a very tiny snail (very, very tiny) or steal some fish food. Maybe in a few years it could take out a nassarius But seriously, it can take a year for a gonodactylid just out of larvae to grow an inch. Target feed it and it will probably be too tame to take down anything. Maybe a snail here and there. Feed it a piece of food 1/2 to 2/3 its size every other day to twice a week. Shouldn't be a problem, and if it becomes one it should be used enough to you to make it a bit easier to catch. Now for a ID I'm just taking a first guess glance, but maybe G. viridis.
