Is this weed or grass seed?
Should I just mow low and cut it off?
Is it POA Annua or desirable KBG?
It is grass seed. It is your KB lawn going to seed. As the seed heads mature, they will spread into a more panicle shape. In fact, it would be unusual for your lawn not to go to seed this time of year. It is not harmful and there is nothing you can do to prevent it (although some have used growth inhibitors to try reducing them). Unfortunately they do nothing for the look of the lawn.
Is this a new to you KB lawn?