Hello Power_Jin, and welcome to the Unreal community.
Your concern relating to the blurriness of the Quixel nanite meshes may well have to do with the import settings. Please right click your texture samples and select convert to regular texture. As noted by member EnjuY1517, if this does not alleviate the concern you may have exceeded your texture streaming pool size. EnjuY1517 continues with their answer on how to increase the value.
I hope this helps.