Can you post a picture?

Planto

Here's a picture

to clearly see it

I'm not sure why it looks discolored, but to me there doesn't appear to be anything wrong with it. If there is, I'm sure someone else will chime in and tell you. It's possible it's just new growth, as newer growth can be a lighter shade. In that case, it will darken over time and you won't even know it was there.

Planto

You might want to remove your Opuntia from that pot, as its roots are voracious and will grow out to the exclusion (and detriment) of the other plant(s) in the pot.

And Beth (Planto) is right - I think that's some lengthening of growth there, and nothing to worry about.