1) When the moon is covered by clouds, if the cloud is so thick that the moon cannot be seen properly, one should not make the blessing. If the cloud is thin and weak some say that one may make the blessing (Mishne Berurah 426). Others contend that if the moon is at all covered even by a very thin cloud that one should not make the blessing. (Chida, Ben Ish Chai Sh”Sh Vayikra 23, Kaf Hachaim ibid).
2) If one commenced making this blessing and in the meantime the moon has become covered with clouds, he should finish the blessing. However, he should not commence the blessing if he estimates that the moon will not remain uncovered until the end of the blessing. (Mishne Berurah ibid)
