This happens when you "Overcharge" your Energy. Whenever you collect Energy from any source, whether it's Portal Energy or Arena Energy, it automatically gets added to your current amount of Energy. In cases where you are already at maximum Energy, it is added on top of your limit and will remain there until used. You will not generate additional Energy until you have used enough Energy to bring you back below your maximum limit.
Why is my Portal/Arena Energy icon glowing green now?
Last Updated: 431d