Skin burn

The OHR lights/sensors are encased in an epoxy resin - perhaps you have an allergy to that?

The other possibility is that the sensor protrusion is irritating your skin.

You could try putting a small piece of clear tape over the sensor for a few days and see if that helps - if so, then I would bet on it being an epoxy allergy, which means Garmin's whole line of products is going to cause the same issue and there isn't really a solution. If not, then the protrusion is probably just irritating your skin, in which case the only solution is to wear the watch looser. Note that the tape isn't a long-term solution to the problem, since OHR sensor likely won't work right through the tape.

A 3rd possibility is that the watch and/or the skin underneath it aren't getting cleaned with soap often enough, and bacteria or fungus are becoming an issue, but I think this is unlikely in your case since it seems to only affect the exact area where the sensor is. In any case, it's a good idea to wash the watch with liquid dish soap (I like to dilute it with water first) to rule that out as the problem.

Yeah, the LED energy levels are nowhere near enough to be problematic - that is almost certainly either a bacterial/fungal issue or an epoxy allergy

Thanks for the reply’s. 
Interesting threads. Definitely a burn on the skin. If it was fungal would be in another area as other threads demonstrate. This is directly under the led lights  

forums.garmin.com/.../burn-skin-from-optical-sensor

There are two rectangular shaped leds so if that is a burn, why shape is circular and much bigger than actual led-lights?

And here is another post with same issues

forums.garmin.com/.../did-my-fenix-5-optical-hrm-just-burn-my-skin-pics-attached

The sensor is raised and in closer contact with your skin. It is also furthest from the edge, and thus has less opportunity to "breathe".

Wearing it firmly and 24/7, it's likely a skin irritation. Wash the watch regularly (especially after a sweaty run), and take it off overnight (or switch it to your other wrist if you still want HR & sleep data).

I had skin irritation with my Fenix 5s plus also first when using but irritations disappeared after i started to keep the strap loose on daily use. Switch the watch to right hand always for the night. And wash it with water always after I shower my self (I take my watch off while showering) Also wash the watch with soap time to time.

I was getting a  ark but took to washing regularly  around the area and also cleaned the back of watch , no further problems ,  my son also has same watch never had issues ,he doesn't wear watch at night and also loser than i do 

This is my 2nd fennix and sons 3rd 

I had exact same thing only on a different part of wrist caused by strap  Its contact dermatitis probably caused by wearing watch all the time.mines cleared up in a few days after not wearing the watch for s spell.