Unknown Coin. Possible Roman? (Id: Diocletian Tet)

It is indeed Roman. Specifically, a Late Roman Provincial tetrdrachm from Roman Egypt. Emperor Diocletian, Year 3 (AD 286-287).

I'm a bit worried about the pale green colour. That's usually either a sign of artificial patina, or a sign of active bronze disease corroding the coin. These late-period tetradrachms were made of very cheap alloy, often high in lead, so can come up in some strange colour patterns.

Grading an ancient coin is different to a modern coin. The "grade" describes the general state of preservation, which has to combine both the original circulation wear on the coin and any subsequent corrosion from being buried for 2000 years. How well-centred it is also affects the price - unlike modern coins, an off-centre strike is worth less, not more, than a perfectly centred example.

Thanx! Any guidance to preservation or stabilization of the corrosion?

Thanx for the info! I read some use olive oil and valve oil. I am thinking it is stable. I have had the coin for over ten years stored with my other coins safely. I can't honestly remember all the detail but I do remember it having color back in the day. I'll have to watch to make sure it doesn't get worse and protect it as best as I can.