[…] the nameless grace (line 8)
The woman remains "nameless" in this poem, so it makes sense that the speaker can't find the words to describe her "grace."
A mind at peace with all below, (line 17)
The woman is "at peace with all below." With everyone on earth? Really? She has no outstanding grudges or disagreements with anyone? This makes her seem almost saintly – no wonder the speaker is in awe of her.