In most modern microwave ovens, it is generally not harmful to keep the display on even when the device is not in operation. The display itself consumes a very small amount of power, and it is designed to be on as long as the microwave is plugged in.
The display of a microwave oven typically shows the time, and it may have some other indicators or features like a clock or timer. These components are usually low-power, and manufacturers design microwave ovens to be energy-efficient in standby mode.
However, if you want to conserve power or reduce energy consumption, you can consider unplugging the microwave when it's not in use or using a power strip to turn off multiple devices at once. This practice can help save a small amount of electricity and contribute to energy conservation efforts.
It's worth noting that if you have an older microwave oven model or notice any unusual behavior, such as excessive heat or flickering on the display, it might be a sign of an internal issue. In such cases, it would be best to have the microwave inspected or serviced by a professional.