An icon is a tiny part of the administrators experience.
Generally speaking admin users have greater responsibility and more available actions than general users so the communication that they are signed in as admin users should be throughout the user interface. In this case an icon is less relevant and could even be wasted space.
For example, an admin user will have more menu items available. They might have a user moderation page and a user verification page. The availability and visibility of these menu items communicates their privilege. Likewise, they might receive messages normal users wouldn't, such as support requests. You could mark these messages in some way that would also help communicate privilege. YOu will often find that most pages can be enhanced to facilitate the job of an admin and this is a better way of letting them know who they are signed in as
This list goes on and in your specific case there will be other examples of how you can use the whole interface to communicate privilege. In the case of a sports team I would imagine there could be special options for deciding on who plays, keeping tabs on desired players from elsewhere, communicating with current players and keeping an eye on those who follow your team. All of these would be unique screens and extra navigation items which would themselves communicate the current privilege.
Another way you could reinforce this would be by using a home button. For a regular user this would just list the followed stream, for an anonymous user this might show an overview and calls to action to join, but for a signed in admin it could show a more complex, specially designed dashboard. Users are used to going home in apps and do it frequently, so this sort of considered redirect would also help communicate privilege