Visibility (something can be seen):
Appearance (looks like / appears):
When something is naturally within one's sight or can be perceived, みえる見える is used. The thing that is visible is marked with が, not を.
This is different from the potential form みられる見られる, which implies having the opportunity or permission to see something (e.g., being able to watch a movie). みえる見える describes something that is naturally visible without deliberate effort.
*(They are naturally visible.)*Compare: えいが映画がみられる見られる。 (I can watch the movie – I have the opportunity.)
Adj-i → く + 見える
Adj-na / Noun + に + 見える
~ように見える (looks as if)