Thank you for posting.
There is no problem with the LED you see there, this green LED is going to be on if the power brick is connected to the NUC and is working. It is an indicator that the power brick is providing standby power to the NUC. We do not recommend to close this since it's part of the air flow of the Intel® NUC.
We had a previous discussion for this matter here:
Intel Customer Support Technician
Under Contract to Intel Corporation