Clams that are open are okay to eat as long as the open clam is still alive.
You can tap the clam when it's open to see if it's alive and if the clam is alive the clam will close again after tapping it.
If the clam is open but does not close when tapped the clam would be dead and should not be eaten because they can grow bacteria in them which can make you sick.
Clams are very perishable and should not be eaten if they're already dead no matter how short of time they are dead.
Clams should only be killed as part of the cooking process so only cook clams after you kill them and if the clams are already dead they should not be cooked or eaten and should be thrown out instead.