One of my Crocea clams hasn't opened up all day. I've had him for a few months and he's usually wide open once the lights turn on. However, I have 1 other Crocea and 2 Maxima clams that are fine. I just did a 20% water change. My water was:

Calcium: 425
Nitrite: 0
Nitrate: 10 ppm (not ideal, that's why I did the water change)
ammonia 0
Salinity: 1.024
Temp: 76-79
I haven't checked my Alk yet.

However, the mantle that I see is still vibrant purple and blue. Do you think there's a problem or is it normal for them to stay closed every once and a while?

I don't know how much temp affects them but I would try to keep it closer to only one degree change tops. preferably about 77-78. I know in nature a few degree changes in the course of only a day can kill entire populations of species.

Make sure that his byssal tissues are attached to a rock. if your clam laying on the sand bad put a flat piece of rock under the sand so he can attach to it. Otherwise the byssal wil become a great target for predators, like worms.
Sudden or slow Ph drops can be also very stressful.
Also check there's might some small crab or snail near close to his mantles.
Try to target feed with live phytoplankton, that might help.

Thanks dick headers.

This clam is sitting on a shelf and he usually has a few hermits running around him, but I will also try to target feed him some phyto feast. I think it may have been the temp change in my tank. It was running at 78-82 and he was fine. I've dropped the temp to 75-78 to control my green algae problem, but he seems like the only one that's been affected. I'll keep you updated and once I get rid of this shitty algae problem I'll post some pics.

Thanks,
newbie