THe green color could be natural, could be algae. Small clams often hitchhike. None that I know are a problem, except that they rarely live long and can spike the nitrates when they demise. Test immediately if you note it not opening or fuzzing up in rot.
Bristleworms and nassarius snails are your best defense against that sort of thing. If your tank maintains a supply of those creatures, they will take care of any unseen demises. If you should get a nitrate spike, an immediate water change will lower nitrates: stay prepared with ro/di water and salt, and of course do your weekly 10% water change.