Obverse: Reverse: Arcadius Follis 392 - 395 A.D. Obverse: AD. D N ARCADIVS P F AVG Reverse: GLORIA ROMANVM Antioch RIC IX 68c
Distilled water soak for a few days to a week. Followed by a soft scrub with that old tooth brush you have laying around. Joke told by a dad: how do we know the tooth brush was invented in Alabama? Because otherwise we'd call it the teeth brush!
Then, I second the distilled water/gentle mechanical scrubbing approach. I think everything else will change the patina color.
As an inveterate coin cleaner, I'd be very careful with that one. From the photo--that's all I have to go by--it looks like the coin has a fragile, green patina, with the brown part underneath. If this is the case, then cleaning the coin and removing the green will remove most of the detail. If, however, the brown is actually covering the green, then yes, distilled water soaks alternated with gentle scrubbing with a toothbrush and dish-washing liquid might be helpful.
Kind of a tough one. AE2 of Arcadius. I've done some cleaning of similar encrustations and also have had my best luck by mechanical means, e.g. water soak, toothbrush, dental pick. I would not try olive oil or liquid descaler which is sometimes recommended.
I've never cleaned any ancient coins before. I personally wouldn't mind waiting for a few more members to chime in. However, at this point, I'm certainly leaning towards not cleaning it.