Hi there im new to this and ive just started a new tank with live rock but ive found a walnut type thing on the rock is this good or bad and could anyone tell me what it is it opens and closes thanks
I don't know what a "standard clam" is? Can't tell much from your photo, sorry. Look at the bivalve in this link. You'll need to refer to this link many ties in the future, so put it in your favorites. LionfishLair.com - Hitchhiker Guide to your Aquarium
By "standard" I meant to take attention off of the Tridacnid clams & place the attention on a Basic hitch hiker. To get to it's exact name I don't think too many people will be able to tell. The picture is slightly fuzzy. It is a Clam or "Bivalve"-Meaning 2 shells.
If you are wondering if it is a threat in the aquarium, No probably not.
I think I can see the outline of the shell. Looks like an Ark. Does it have red meat on the inside? Did your rocks come from Florida? Because I'm thinking Blood Ark or Ponderous Ark.