Forward by 1: f →g, y →z, l →m, m →n → g z m n → no. Backward by 1: f→e, y→x, l→k, m→l → exkl — no.
Alternatively — it could be ? Try swapping adjacent letters in "fylm" → yflm? No. fylm sl aswd mtrjm anjlyzy
→ "slyz fy nfjq zgewz nawylml" — nonsense. Forward by 1: f →g, y →z, l