memory

If you are correct that your computer is only seeing one memory card, I suggest that you check the one that is not recognized. I would remove it, ensure that the slot is clean, the connecter edge is clean and then reseat it, making sure that it is fully inserted. Otherwise you may have a card that the Mac can not recognize. Are they from different providers?

I agree with @Ronasara and would add that if the one module is actually bad that you remove it completely. It is possible the failure could get worse to where it could affect system performance or even prevent the computer from booting.

yes