Prerequisite requirments I want to start research in the field of Computational Cosmology. It includes the galaxy formation and reionization era. I know it involves extensive computing techniques including parallel and high performance computation but do I need to study General Relativity (detailed course) or I can work without it (or having its basic concepts)?
For the physics part, I have to study Astrophysical Hydrodynamics and galactic compact objects and their X ray sources etc. 
Do I need complex differential geometry of General Relativity for this? 
P.S: I have a physics background and right now in course-work phase at graduate level in Computational Science degree.
I don't know if it's 100% necessary for computational cosmology but I would highly recommend studying General Relativity if you can, especially if the course includes some cosmology as well (fairly common for undergraduate and graduate courses in GR). 

Fluency in differential geometry is (I think) an essential skill for a cosmologist, computational or otherwise. Studying GR and cosmology will also greatly help your understanding of the bigger picture, for example, the FLRW background, the Friedmann and continuity equations etc. 

Plus GR is a really beautiful theory! I would recommend that any physicist study it if they have the chance.