I haven't had a plating coral in a long time, but I remember that sometimes detritus could build up on a spot and cause that back when I had some. Also if you add supplements to the tank on top such as calcium it can get on a plate before its dissolved. Those are a couple things I can think of.
Hard to see but it could also be growth, my cap formed a few lumps on it... If the spot gets bigger in diameter than it may be problematic. Coloring appears to be the same as the outer edges. One thing is for sure you will find out in no time if it is an issue.