If the AP is grey in the Meraki Dashboard that indicates it is "dormant" which means it's been offline for an extended period of time and has not checked in to Dashboard. However, it is possible the AP is still up and running, and when the LED is blue, that means there is at least one client is associated to at least 1 SSID, and when the LED is green that means the AP is "ready" but there are no clients currently associated.
You'll need to check into why the AP is unable to check into Dashboard, could be a local LAN or VLAN issue, or a firewall blocking its outbound connection attempts to the Dashboard for example. If you think that might be it, if you're behind a restrictive perimeter firewall that even blocks outbound access by default, check under Help > Firewall Rules to confirm your firewall is allowing the proper access, such as UDP/7351 for Dashboard communications.
On the idle user question, the AP will not disassociate a client just because it's idle. How many clients did you want or expect to connect at once? If that's a large enough number, and depending on what the clients are doing, you would typically add an additional AP. If the client is still associated but otherwise idle, it wouldn't pose much of an issue anyway, but there are many variables to consider.