Tube turning black

Hi all,
I have a big bottle RCA 5Z4P in my preamp.
I noticed tonight it has turned black from the base,up to the middle of the glass.
Its a haze like blackness.
I dont remember ever noticing this on other tubes.
Is this normal?
If this is an issue,then what would cause this?
The preamp seems to be fine.
The other tubes in the preamp are 12aT7's.