As noted, I've never seen anything like the bubble blister on your FH before so I did some digging. The problem might be a "gas embolism" or gas bubble disease. Somehow excess gasses in the water make its way into the affected fishes system causing bubble blisters often times on the head and eyes. The articles I read advised not to pop the blister but to correct the gas imbalance in the tank. Good luck identifying and solving that situation. The only other possibility is lymphocystis. This virus causes blister growths but usually not perfectly rounded and smooth like on your FH. Lymphocystis is also called cauliflower disease due to the bumpy texture of the sores. I hope this points you in the right direction.