Poa Trivialis (aka Rough Bluegrass) is similar to Poa Annua but is harder to kill. It leaves a thick, light green mat of grass which can choke out the lawn. It dies off in the summer heat and can look like grass getting a fungal disease.
There is no selective weed killer for Poa Triv. Some have reported using Tenacity can damage or kill it, but it is not labeled for that use so your results may vary.