What does the green halo around Wizarding Challenges means?
Solution 1:
The green aura itself is purely cosmetic and does not affect the gameplay. When you come across a tower with this aura, it's to indicate there is a challenge either in progress or recently completed by you or another player. If there happens to be other players around, then you and the other players can team up to achieve better rewards.
Sources
- EuroGamer
- iMore