Lea/Leah is the old Anglo-Saxon for 'meadow, field' (lea is still used as a synonym for 'meadow' here in England) and everyone who referred you to Marley was spot-on correct. Marley is made up from the elements maere 'boundary' and leah/lea meaning 'meadow' so 'meadow near the boundary' - it's an English placename. Spellings are never static and in the times of general illiteracy and slower-moving sharing of information they were even more varied - the same place might occur in various records as Maerlea/Marlea, Maerleah/Marleah, Maerleas/Marleas (pl), Marleigh, Maerleage or Marley - the elements and intentions were the same but the spellings were not.
Just finding a name on the internet is of no consequence - yes, it exists and isn't that uncommon but that doesn't mean it's completely unconnected to another name. You'll find tons of 'Mandy' out there too but they all lead etymologically back to Amanda, regardless of whether or not the parents make the leap of logic or recognise it on the birth certificate. Marleah is simply a varient spelling of the placename/surname Marley and could indicate that it was once a surname in your family or that someone in the line hailed from one of the places called Marley/Marleah - here in England you'll find them in Devon, Kent, Sussex, West Yorks, Ches and over in Co. Durham and there are probably dozens more in America to check out also.