Word for shaking elbows (Covid-19 pandemic)

It's called elbow bump.

Elbow bump: A gesture (usually of greeting or farewell) in which two people lightly tap their elbows together as an alternative to a handshake or embrace, esp. in order to reduce the risk of spreading or catching an infectious disease. [OED]

It's been shortened to elbump.

According to OALD:

Elbump (noun): An act of briefly touching somebody's elbow with your own to greet them, done to avoid spreading disease through touching each other's hands.