I have st. Augustine lawn. There are runners of st. Augustine (at least it looks like st. Augustine) over the top of established lawn. Mowing cuts back somewhat but looks terrible. Have never seen this before. Also patch where grass is thinned out Thanks B Langston
I can't make a determination from your image.
I would suggest taking a sample of the weed to your County Extension Office for identification.
Here are some links with more information including a link to locate your nearest extension office.