Matcha cheesecake can't keep the bright green color
Lemon juice / exclude air.

If gray outside only but inside is good I conclude the matcha oxidized from the air.  I am not sure why, but if it stayed green inside it must not have been any ingredient - they are all inside too.

Ideas to prevent oxidation are either preventing air from coming into contact with the cake, or adding antioxidant.

Some apples (not all types; not sure why) turn brown in the air.  Lemon juice prevents this.  I think it is the vitamin C in it acting as an antioxidant.  You could squeeze lemon juice over the top.  Lemon juice brings that flavor to a thing.  It seems to me like it might be OK on this.

You could cover it tightly with plastic wrap such that air is excluded.  

Best of all, if you have any cake left,  would be if you would do an experiment with both methods and post images.  Take out a slice of cake, cut it in half, treat 1 side with lemon juice, then new pic next day.  Also take a slice of cake, cut it in half, cover 1 half tightly with plastic wrap and other half leave out.  New pic next day.