There are different varieties of St. Augustine, with almost a factor of two difference in blade width.

One problem for us is you took the pictures from a distance and in direct sunlight. The Sun reflects bright and casts a dark shadow making the contrast too much to see anything. Here's the difference between high contrast and low contrast.

If you could take a picture as the Sun goes down or on a cloudy day that would be better.

It could be nut sedge which is almost impossible to eradicate. I've been fighting it for years. The chemicals sold specifically for nut sedge have not been effective.

Nutsedge can be controlled by proper watering. Nutsedge is a swamp type plant that loves to have "wet feet," or saturated roots. When you take away the water for a week at a time, the nutsedge goes away. Way back in the 00's our city replaced our water meters and refilled the hole with soil containing both bermuda and nutsedge. All I did was normal (deep and infrequent) watering and both weeds went away.

At this stage (St Augustine not well established) there's nothing you can do to kill weeds that wouldn't kill St Aug first. Nutsedge is about the only weed that St Aug won't smother out once it's established. To see if plant is Nutsedge roll stem between thumb and fingers. Other weeds have round stems,nutsedge stem is destinctly triangular.

just wanted to comment that most "weeds" that they tell you are weeds are edible and nutritious for you. like dandelion. it actually helps your soil...and you can eat it all kinds of ways. others are natural pain relievers.... etc. others you you can just eat...free food.