It appears to be, but is hard to tell from the angle. When you look down at your clam, when the mantle pulls back in is the inside of the shell there along the rim white? If so, it is new growth. It can be pretty easy after awhile to see the new growth as the old shell looks more green/brown where the new is bright white. Like I said, it appears to be growth.
Can you tell us more about it? It is a really nicely colored Derasa. How big is it? How long have you had it?