I cannot see the features right, When on the gloom I strive to paint The face I know; the hues are faint And mix with hollow masks of night;
Cloud-towers by ghostly masons wrought, A gulf that ever shuts and gapes, A hand that points, and palled shapes In shadowy thoroughfares of thought;
And crowds that stream from yawning doors, And shoals of pucker'd faces drive; Dark bulks that tumble half alive, And lazy lengths on boundless shores;
Till all at once beyond the will I hear a wizard music roll, And thro' a lattice on the soul Looks thy fair face and makes it still.
What could be worse than not being able to remember what someone really important to you looked like? Probably not much.
This is what's happening to Tennyson right now. He's having a hard time remembering what Arthur looked like. This is a common thing that happens when someone close to you dies—or even if you just haven't seen someone in a long time. You have a difficult time remembering his or her face.
It's only at night (in dreams?) that Tennyson is able to really remember his friend's features clearly.