merfolk cheek "fluff" is one of those details that i'm deeply in love with but find a hard time bringing it up or mentioning it. in essence, it's the same structure as the larger "fin" (really a modified external gill), just without a "quill" that it branches off of. they're very thin, very delicate strands of skin that develop around the base of the fins and lead into them, helping to "smooth off" the area as it transitions and provide more detailed sensory information. they aren't as mobile as the larger fins, but they can still rise and fall and modify the shape of the fin, and can be used for some finer merfolk expressions.
being modified external gills and covered in delicate skin that's capable of a minor role in oxygen exchange, they're fairly fragile. merfolk beauty standards in the past have prioritized whole, undamaged fins, but in the modern era, risk of damage is much easier to avoid, and damage that is obtained is easier to heal. still, considering the amount of nerves in the same area, merfolk will be highly choosy in who they allow to touch the "fluff", and being allowed to do so is considered a high mark of trust.
that said, it does feel nice when someone scratches just that right spot.