Hi, Korann-
Well, that looks like it could just be the CSF in the midsagittal part of the brain, possibly, that is showing in the sagittal slice.
I don't know what viewer that is, but the representation looks a bit difficult to interpret. It would help to see it in the AFNI viewer (for familiarity's sake), preferably overlaid on the template you are using or on the anatomical that has been aligned to the template space (probably called final_anat*HEAD in your *results/ directory from running afni_proc.py). You can look at it in different slices, too, to see if that is the case.
--pt