It's not Ramuh Materia (or any materia for that matter) and the spelling of stile on the last line is completely intentional.
It is definitely an easily missed item (having missed it myself before!) and it is in an area that you can't return to.
The biggest clues probably come from the second and fourth lines. The third line is obscure but relates to the item. There's plenty of clues in there if you look deep!
Edit: here's the riddle again as we're on a new page...
I'm easily missed, though I have no lock
You'll see I come from a rough sea stock
Critters may come and give thanks for a while
As I sit and weigh up Avalanche's stile