Another picture from under our birdfeeder of sunflower seeds, in an untended wood mulched area.

Is this just a weed grass? Or some sort of grain (based on the top - the seed head?)

It got big quick! I wish my real grass lawn were this big and healthy

It reminds me of a foxtail grass, Setaria. To identify it would need closer views of specific parts of the plant. Compare it with the three foxtail species characteristics in the link below:

http://www.extension.umn.edu/a...

It looks like what we call bristle grass.

Bristle grass is another name for foxtail grass (Setaria):
https://en.m.wikipedia.org/wik...

Thanks!

Yep, looks like a variety of Setaria (Bristle Grass): Green Foxtail (Setaria viridis)