Why has my zirconia crown become gray? (Photo)

Hi .i had all my teeth done back on May. After 4 months 1 of the crown become darker (see pics).there is not pain and i did not have any pain at all.now when i heat i feel a bit when you compare with other teeth.this teth holds a bridge as well. So i have done fresh x ray to have a look and seems like i do have a lot of infections. I don't have no pain at all. Can someone explain how this can be treated.also is this very bad and do i have to worry obout?? I will add x ray before and now to