And this is why, if you're running with a Genji; Brigitte, Lucio, Ana, and Zenyatta are probably the better heal options for your team comp (all of which either have reach to heal, or heal passively so you don't have to follow the Genji around).
Genji is sort of tasked with being all-freakin-over-the-place, and he will lead you through a demilitarized zone in the effort to be a pesky, ult-building, shuriken-spamming mosquito. It's just how he's built to play--he needs to get up in faces to land those point blank fan-star + swift strike 200hp-kill combos, and to deflect into the faces of certain enemies like Bastion and Orisa. Do not follow him in normal circumstances. It is not in your best interest as a Mercy player. You will get out of cover and die a lot in the process.
Mercy's big contribution with Genji is mainly just to boost on blade so he can one-shot 200 hps, in absence of an Ana nano-boost. Preferably, with Valkyrie so you can stay out of the way. Sustained Mercy pockets for healing are better left to snipers like Hanzo, Widowmaker, and McCree, and to Pharah, who will all be able to do their job without 'constant' ingress into enemy turf, while playing spread out enough to still give you the benefit of a safe spot to fly to, from guardian angel. Resurrect is also much easier to play from a sniper's perch (or wherever the hell Pharah crashes and burns), after they lose a duel, than directly in the middle of whatever deathball Genji was trying to get out of.
TL;DR Genji's long-term survival is not Mercy's top priority, leave it to your other support or pick a different healer if his life expectancy is lacking.