The Mercy player appears to have adopted the bold strategy of standing stock still on top of a pillar in Temple of Anubis probably a little too secure in the knowledge that the other team doesn't have a Pharah.
Across the gap is their team's Ashe, who Mercy has seemingly decided is the only player on the team worthy of heals today. At one point the Ashe somehow dies despite the constant healing and buffs and Mercy flies off, only to come back seconds later to revive Ashe with her Ult. Then either by some miracle or by the fact that they were standing still the entire time, the Mercy gets taken out by the other team's Hanzo and is left in a bit of an undignified position on the pillar.
Playing it safe is all well and good but getting comfortable in any position (especially one that's in view of snipers) is a bad idea. While it is generally better for healers to stay out of the direct line of fire, it doesn't mean they have to just stick to being the heal bot for a single teammate the entire match either.