Both dramas are similar in the aspect of ghost seeing, supernatural elements and ghosts staying back instead of crossing over to protect their child. Even after death the main leads who are dead and now ghosts stay back on earth and protect their daughter from harm and resolve their regret.
The difference is in "Hi Bye, Mama !" the female main lead a.k.a mom ghost gets the opportunity to become human for 49 days and accompany her daughter whereas in "Who are you?" the dad ghost can possess the male main lead and help his daughter from behind the scenes without telling her that he is the ghost of dead dad who possessed the male main lead.

Both have similar phenomena such as space and time displacement creating a time travel effect.In both the dramas Female main lead comes from the future in order to complete a mission

Both Love, Again(2019) movie and Emergency Couple (K drama ) is about how the main leads get divorced due to their unhappy marriage life but later meet and start falling for each other again

K Drama with a similar story of how MML was in love with the FML from high school but never confessed and how both MML and FML were in relationships with others but never worked out for them .

Both have a common aspect of sci-fi and male lead being in an accident leaving him unable to live a normal life.
In Where Stars Land (2018) the male lead has a disability aid helping him move his arm and leg.
Whereas in Miss Crow with Mr. Lizard (2021) the main lead met with an accident leading to a life-threatening injury to his heart forcing the surgeons to perform an emergency operation and implant a mechanical heart.

Both dramas deal with time travel / time slip.
Manhole(2017) - Male main lead Bong Pil time travels into the past through a manhole in an effort to stop the wedding of his lifelong crush, Soo Jin, set to happen one week into the future. whereas in
Blue Birthday (2021) - Female main lead Oh Ha Rin will revisit the past through mysterious photos left by her first love, Ji Seo Jun, who died on her birthday 10 years ago, and prevent his death. (for every time travel the person needs to pay a price )