Sure, I'd be happy to tell you! (I probably should've explained in the game, oops...)
Fey'lana is "faerie maiden" or "faerie lady", but has the same meaning as "female" or "woman". Though it's a technical term for a female faerie, it's commonly used as a term of endearment by both parents to their daughters and fae to their female lovers.
A is "my" and maith means "good". So a literal translation of "a fey'lana maith" would amount to, "My good girl." But "maith" in the fae tongue is not a mild world. When something is "maith", it carries a feeling of "it doesn't get any better."
So the meaning of "a fey'lana maith" in the fae language is more like, "My perfect faerie lady." However, for it to be a purely sweet and loving comment, the phrasing would be simply, "Fey'lana maith", which would imply "you are perfect." Instead, because it's Aeden, he added "my", which gave it a decidedly possessive and sexual spin. Still sweet, but also spicy.
I should go ahead and tell you that you'll soon come upon the phrase "Fey'dorna sith". Fey'dorna is "faerie thorn", but basically has the same meaning as "bitch". Sith is another term for "faerie" but it refers to the Solitaries, so it implies "corruption".
So when Aeden says, "Fey'dorna sith," he is essentially saying, "(You) evil bitch."
All that might've been more than you wanted to know, haha, but I hope it helps!
EDIT: I'd like to add that "dorna" as a standalone word just means "thorn" and has no negative connotation. Adding Fey to it is qualifying "dorna" as a living thing (or person) and not an object, and so the meaning evolves to that of "bitch."