Need help identifying this

It is very difficult to see, but could possibly be Silverweed ?

 

It's very difficult to get rid of if its been there for years, there will be seeds in the soil so even if you kill it off with chemicals they will come back. If it's only a small lawn then you can hand weed them. If bigger you could try a lawn specific weed-killer, one that contains 2,4-D but it will need several applications. Professional lawn care companies have access to stronger weed-killers and would be able to advise if it really worries you. You would then need to feed and mow regularly to ensure it can't establish again.