Is this Japanese stiltgrass?
This is not Japanese stiltgrass, which you can see here: https://extension.umd.edu/hgic/topics/japanese-stiltgrassa
Instead it is a type of Smartweed (Polygonum pensylvanicum). Here is our page on it: https://extension.umd.edu/hgic/topics/smartweed
There are some that are native and some that are not. You can search "smartweed" types and compare on this page from Maryland Biodiversity Project. This may be the non-native invasive: https://www.marylandbiodiversity.com/viewSpecies.php?species=16469
Thanks so much Christine! How can I tell the difference between the native smartweed and the invasive smartweed? I've tried looking at pictures online, but I don't really see a difference between the two. Are there any key characteristics that I should look for that will help me tell more definitively which one I have in my yard? I think it's a very sweet little flower, but I don't want to let it stay if it's invasive.
Probably your best way to identify the Oriental smartweed is by its hairy joints. Look closely at the photos in the Maryland Biodiversity link above. Look for those hairs. The reddish areas on the stems may be a clue, too.
You photos don't show joints vert clearly, but what we see seem to be clear of hairs. We recommend that you examine your weeds yourself, however.