What's Up with my Black Moor? All of a sudden he (or she???) sits behind tank ornament?

I have a 29 gal tank with 2 black moors and a couple of Fantail goldfish in it. It is decorated with quite a few artificial plants and a few ornments (for hiding etc). They have all been doing great but now all of a sudden one of the black moors has began sitting behind the tank ornament for long periods of time. He/she will only come up to eat and very seldom swims around. I am wondering if there is a problem? OR is this somewhat normal. He looks fine - water tests are great.