Seahorse death clear mucus found

[swill]

Several months ago I lost a Lined Seahorse. He was eating great, showed no sign of stress or disease. He was in quarantine as I had just got him a week prior but I honestly thought he was doing great. I woke up one morning to him still attached by his tail to a rock but laying flat. When I scooped him out I found this clear mucus attached to his head. I have not been able to find anything online so was wondering if anyone could tell me what this might be from? Here is a picture, sorry if a picture of a deceased Seahorse upsets anyone.