Growing on shag bark hickory

What are these? Noticed them on the leaves of my shag bark hickory behind the house on the edge of the woods. I first noticed them this past Saturday and they were a pale yellow/green color, didn’t have time to take a pic. When I walked back tonite(Thursday) they now look like this. Thanks.

 

They're leaf galls. You know that bump you get on your skin when a mosquito bites you? This is a similar response by a tree to insect damage, although bacterial and fungal damage can also cause the response.
By the time you see them, the guilty organism is usually long gone. They're not egg sacks or anything like that, it's just some deformed cells formed as a reaction to being chewed on. They won't harm a healthy tree. On hickory trees, they're usually caused by aphids.

 

Looks like hickory leaf gall. Dissect one and see if there are immature aphids inside.

 

Thanks.

 

Thats one of the harmless ones. No worries. My neighbor has some nasty ones that cause tip dieback.