In fact it is not the cap that is green. The rubber under the cap that is green when mineral oil is used. (Mostly)
You can double check with a drop in water:
1 dot oil mix well with water and turn white.
2 mineral oil floats
Seems ok to me.
If it is normal, sound should keep average the same as time goes
If there a problem, it probably will get worst and worst.
So, give bike a good ride, have fun and hope everything will be fine