Q.Cerastium Was A Tight, Soft Green Mound This Spring, It’s Trailing Now And Yellowing At The Center.
does cerastium need to be sheared in early summer when it begins to trail and yellow at the center?

Certified GKH Gardening Expert
No, this will indicate a problem with your soil. They prefer things on the dry side, so I could suspect that the soil is too moist. This could cause infection, or just root suffocation, but unfortunately, your photo did not come through. I am unable to see what is going on.
Generally, testing your soils for nutrient issues, or pH issues will tell you all that you need to know. If everything is within range, then you will know that you need to treat for disease.
Here are some articles that will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-moisture-in-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/test-soil-for-pests-and-diseases.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-sure-soil-drains-well.htm
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm