DIAGNOSIS The washer's reminder system is alerting you that it's time to run a cleaning cycle. It's a timer-based reminder, not a sensor.
FIX Run a Clean Washer cycle. Select the Clean Washer cycle, optionally add a cleaning product or bleach, and press Start. Let it complete. After completion, the light should turn off automatically.
IF THAT DIDN'T WORK There might be a control board glitch. Unplug the washer from the wall outlet for a full 60 seconds, then plug it back in. This performs a hard reset and often clears the reminder light if the cycle didn't.
HEADS UP Ignoring this can lead to mildew smell. Run the Clean Washer cycle monthly if you do frequent cold washes or use lots of detergent. Leave the lid open between washes to let the tub dry.