I strongly believe it is not crying. It forms a visible eye on its face here, but no liquid comes from there. The liquid comes from the bottom of the head, and it’s not spaced out enough to suggest eyes, I’d say. Are we supposed to think that there’s a gap in the bottom of the helmet with black tears coming from the knight’s real eyes dribbling out?
And, if it were melting, on the other hand, why do the drops seem to come from below the face rather than being a part of the face itself, and why does only a single spot on its body melt?
I think the knight is drooling/frothing at the mouth after its savage attack on Tenna, which seems to me to have been more motivated by anger than strategy