The Torah doesn’t tell us what she saw in him that was ‘good.’ Chazal tell us that he was born with a Bris Milah already performed, but this is a Drash, not the simple explanation. Additionally, why does this seem to be the reason for his mother’s hiding him? Had he not been ‘good’, would she not have hidden him from the evil ones who wanted to mistreat him?
The Ohr Hachaim explains that the Gemara explains that the Mitzrim counted the weeks and months from the time that Amram married her, in order to figure out when the nine months were over. Yocheved gave birth to Moshe after six months, and the Mitzrim were unaware. A baby who is born with only six months of pregnancy cannot survive; he needs the seventh month too. However, women can make a mistake about one day, and Yocheved thought that Moshe was born in the sixth month, not afterwards.
When Moshe was born, Yocheved expected him to be a frail preemie. However, she saw him and he was 'good’. He was strong and fit, not weak and frail. He was obviously born in the seventh month, and he would survive. She hid him away, because she knew that the Mitzrim would give up on him as a frail child. As long as he was hidden, he could survive.
However, because the Torah uses the word טוב – good, Chazal deduced that this teaches us that he had a Bris Milah.
This means something deeper too. The Gemara tells us that Adam Harishon was born completely pure, without the option of any evil. When he ate from the tree, he added a foreskin that symbolizes impurity, and now he needed a Bris. Moshe was born completely pure.
