You are right. It's a phenomime and adverb, meaning, sorry that they are adjectives, misty, hazy, foggy, murky, gloomy and unclear. Usually the word is written 'もやっと' with a small tsu/っ. The line can be translated as 'It something like your heart is shrouded by fog...isn't it?'  If you have dictionaries look up the word 'moyamoya/もやもや'. The meaning is almost the same. When you need Japanese help, don't hesitate to ask me.