Hi Minji,
After the DWI preprocessing and FreeSurfer, there is an additional step that is required which may have been missed. The anatomical images (outputs from FreeSurfer) need to be aligned with the DWI, although mrgrid won’t do this. Check out this post on how to do that
After the images are aligned, do mrcalc and mrstats to obtain the FA in the hippocampus (be sure to use images that are indeed aligned 
)
As for hippocampus tractography, this paper might be helpful New insights into anatomical connectivity along the anterior–posterior axis of the human hippocampus using in vivo quantitative fibre tracking | eLife
Hope this helps!
Cheers,
Arkiev