I am seeing this grass all over the lawns here in Southeastern Pa. Any idea what this is?
Based on the timing of the grass's appearance, its color, the alternate/nearly paired arrangement of its blades, and its form, you have crabgrass. The base of the grass's blades is often a purplish color, which is also an ID key.
Here's more information on identifying crabgrass. Note that the first photo in the attached link shows a much more mature clump than the ones in your lawn.