Hmm. Maybe you could make up a story? Say, you're shedding your old scales so new ones can grow in in time for spring Or maybe temporarily losing some scales is just what happens when mermaids spend time on land, and that's why you don't see many of them Or you can always just say that's the way they grow. No one can tell you you're wrong!

Up to you, whatever you decide--I'm sure it will turn out lovely.