While more hard-core Marans folks may differ (and will most certainly chime in, LOL!), I personally would hang on to an all-white "sport" to see what happens if you have the space. If you get a couple across several batches you might even be able to see if they breed true. If not, you can cull to someone else's backyard flock - - someone might just love having a white Marans. I would not keep in a regular breeding program with the others, though.
If you got the eggs from a breeder, I would let the breeder know just out of courtesy. A serious breeder would want to know of sports popping up from their breeding program.
Hope you will share photos as the chick develops. Good luck!