Hello TSMSMC,
If the heart icon on the workout progress screen is gray, that means it it unable to determine your heart rate at that moment. If the heart icon is red, that means it is actively tracking your heart rate.
To ensure the most accurate readings, make sure the watch is fitting securely, but not too tight on your wrist.
Monitor your heart rate with Apple Watch
If you have the Low Power Mode option turned on, that will turn off the heart rate readings so in your workout progress the heart icon will be gray.
Use Low Power Mode on your Apple Watch
Cheers!