First things first... the light emitted by the sensors is rare unless doing a workout and when I work out I'm usually running/jogging so I don't get bit very often while jogging. Let's look into this... what type of light does Apple Watch use? But first - can mosquitos see infrared?
According to:
https://sciencing.com/animals-can-see-infrared-light-6910261.html
"Blood-sucking insects, such as bedbugs and mosquitoes, rely on their infrared vision to feed themselves. They can "see" body heat and use the heat signature of the carbon dioxide (CO2) gas that humans and other animals naturally exhale to locate their prey."
According to:
https://support.apple.com/en-us/HT204666
"The optical heart sensor in Apple Watch uses what is known as photoplethysmography. This technology, while difficult to pronounce, is based on a very simple fact: Blood is red because it reflects red light and absorbs green light. Apple Watch uses green LED lights paired with light‑sensitive photodiodes to detect the amount of blood flowing through your wrist at any given moment. When your heart beats, the blood flow in your wrist — and the green light absorption — is greater. Between beats, it’s less. By flashing its LED lights hundreds of times per second, Apple Watch can calculate the number of times the heart beats each minute — your heart rate."
So... the exercise light (green LED) is not infrared.
However, the infrared light (from Apple site above):
"The optical heart sensor can also use infrared light. This mode is what Apple Watch uses when it measures your heart rate in the background, and for heart rate notifications. Apple Watch uses green LED lights to measure your heart rate during workouts and Breathe sessions, and to calculate walking average and Heart Rate Variability (HRV)."
With this information, I conclude that the mode that Apple uses to measure your heart rate in the background would occur so infrequently that your body heat and carbon dioxide that you exhale would be far bigger of a target than a very infrequent background infrared heart rate check.
My other observation is your comment that you got bit UNDER the watch? There isn't a lot of space for a mosquito to fit under the Apple Watch. Most mosquito bites (and I have a blood type that mosquitos prefer) are on open skin like ankles in the summer or back of neck, not in hard to reach areas.
