Ok, it's not really dandruff, but it's sort of similar. This is a piece of skin from an adult female humpback whale, the mother of Noah...our 24th ID-ed calf for this season.
Most of the time, when a humpback is active at the surface, it sheds bits of skin like this. So if I were a DNA-specialist, an easy way to get samples would be to jump in the water and collect the samples by hand.
In case you're wondering, the skin is tough, a bit of a cross between leathery and plasticky (is that a real word?), supple while underwater, growing more brittle as it dries out.
When Noah's mom dived down, there were hundreds of skin flakes like this floating around. Maybe she needs to try a new brand of shampoo?