녹다 is melting or dissolving: a body thawing after the cold wind, meat that melts softly in the mouth, someone joking that he doesn't melt easily. Figuratively 녹아 있다 means something has been dissolved into a work so it is part of the fabric — an identity, a message, a theme running through a film without your noticing. The -게 slot gives the manner: 부드럽게 (softly), 예쁘게 (beautifully), 자연스럽게 (naturally).