Red Symbol on Server Roles
Zooming in on the icon it looks like a red push pin.

Searching specifically for red pin and management studio I found this thread which explains

If a principal is disabled, then it has a red arrow pointing down in
the icon ... Here you have a red pin icon and
this means you can't modify it. Since SQL Server 2014 we can create
user defined server role ... which
wasn't possible in SQL Server 2012 and lower version ... user defined
roles don't have a red pin and can be modified, while predefined
server roles do have a pin

One exception is that the permissions of the public role can be altered (but membership can't). For all other pre-defined server roles the reverse is true.
In more recent versions of management studio it appears a lock icon is used for that instead.