God be merciful unto us, and bless us. An echo of the priestly blessing (Numbers 6:24, Numbers 6:25), but not necessarily uttered by a priest. The substitution of Elohim for Jehovah is natural, considering the universalist character of the psalm.
And cause his face to shine upon us; literally, with us. "With us" especially, as the people of God; but not "with us" exclusively, as the whole psalm makes manifest.