i think thats like a little scrape. one of mine has just one rough scale. im not sure what she would have scraped it on. she has a clay pot base (with and entry hole hammered out of the side) and i think thats what it was.
my columbain rainbow recently made its way into a tiny hole on the bottom of my bath faucet. it went in until it couldnt fit in anymore, then a little farther. now the area that was a little fatter then the hole is dull and rough. so i think he scraped off the top layer of skin.
like Will said, it should go away in a shed or two.