The extended halo around NGC253 is real and well documented, you have captured this very well.
But.. sorry, I suspect your flat-fields need re-calibration.
There is vignetting and flux at 2 o'clock in the image you posted: likely this is an instrumentation artifact ( along with the bright corners from the vignetting) .
There is also an interesting s-bend looking feature at lower left!
Could be you have discovered a new neb..or might be an instrument artifact.
Attached your posted image with the proverbial stretched out of it...