HW97kt piston seal

The piston seal on that gun can rotate and it appears that yours has been. Each little black spot is from where the hot gases rush from the compression chamber through the transfer port, to the rear of your pellet when you fire the gun. If a guy over lubes and it gets in front of the piston seal, the hot gas can actually cut a channel across the face of the seal (don't ask how I know). Yours doesn't look like it has that problem. 

On whether or not it would be replaced......up to you. Is it shooting up to specs? Do you like to experiment and see what effect changes have? I kinda lean to recommend replacing it, not cuz it's no good, but cuz you've already got it apart and if you put a new seal in it, it'll be that much longer before you need to go inside again. 

I really like the Jim Macarri Hornet seal but it's kinda tricky to size right. 

Lube excess can produce that thing called diesling, I sugget get a new one but you should clean and relube properly in order not to have the same problem again

Except for the discoloration, that piston seal appears to to be in excellent shape. Clean out any excess lube and clean off the seal and you should be good to go.

R