Last week I received a Citizen watch that appeared to be in almost NOS condition. When it arrived I was slightly disappointed by the dull dial and hands, they just weren't very bright, which for an orange dialled watch isn't what you'd expect.
here's a picture I took yesterday (it was sunny!!)
I took this angle to show off what I could see, a kind of dusty crystal on the inside, making it appear more like an acrylic one. The dust was sparkling in the sunlight.
I'd seen this once before so I was fairly confident that if I could clean the underside of the crystal it should improve things.
So last night I took the back off (a nice screw back), to reveal an all metal movement. To get the movement out I had to remove the three pushers along with the crown. Once these were out the movement came out easily and I could clean the inside of the crystal with a cloth.
As soon as I had given it an initial wipe, it was clear it was going to clean up nicely.
Here's todays picture:
with the crystal living up to its name and the dial looking much clearer and brighter.
I can only assume that the deposit is from vaporised lume which has degraded over time.
It is probably a combination of lume dust and vapourized oils. This was common on some older model Citizens and even some Swiss watches. As you discovered, it is easily cleaned up.
The fogging from oil vapours can be an ongoing issue with some Chinese watches. My Alpha sub has to be cleaned once a year. Fortunately, the deposits get less and less each time. Maybe when it doesn't fog any more it will be time for a service