Here’s a helpfull answer from Purple Air staff :
"The sensor will appear gray on the map if it is disconnected from the Wi-Fi. It will disconnect from the Wi-Fi if there is an issue with the Wi-Fi connection or it loses power. So, if the sensor is gray, it could be any number of things. Such as the power supply not receiving power, the power cord becoming faulty or disconnecting, or the Wi-Fi connection.
If the dot has disappeared, it’s likely the option selected on the PurpleAir map. If you click on the gear in the top left-hand corner of the map, there is an option called “Reporting or modified within.” This selection will show any sensor currently reporting or if it had been online within the selected time frame."