is this a baby clam

and whats wrong with it this happen in one day

Dunno about the baby clam.

What kind of lighting do you have?
What are your tank params?

Its dead. water params and lighting are key to keeping a clam alive and well.

150 watt metal halide 14k everything was good exept nitrate was 20 and ive haded for about 7 months

its not dead, it can still make it. just do a water change and add alot of carbon to your system and watch it.
did you just add any phosband or anything? hes really stressed and could be from water most likly. but could be from a fish or coral stinging it.

as for the baby its not. its just another by-valve thats been on it for god knows for long. keep us posted. try the water change and carbon, carbon is a clams best friend.

their was some dasy polyps that greww right in to it and hair algea i scrubd the algea with a tooth brush

Its either an ex-clam or soon to be one. They usually don't recover when they look like that.

I too would have to say that at this point it is toast. If the clam is no longer responding to light/shading (i.e. closing when you pass your hand over it creating a shadow) then it is most likely already dead. The tissue does not look alive and healthy; it looks like the clam has already died.

Personally I would take it out to prevent it from decaying in the tank. If you have any hermits, bristle worms, etc. they are going to have at it real soon.

Best of luck with your future clam endeavors!

freshwater dip it.

at this point a FWD will probably stress it to death.......sorry to say dude it doesn't look good....but having said that what do you really have to lose with a FWD?? not much