what is this little green clam-thing?

[dug5921]

i bought a dozen hermits a couple of weeks ago, and i always get a few empty shells for them to grow into. i saw a little (size of a corn kernel), lime-green thing (?) inside of an empty shell. didn't think anything of it at the time, but when i can home today, it was crawling on the side of the tank. it looks a lot like a tiny quahog (clam) with a few small hairs on it. is this bad or good??