It's not a stomatella. Doesn't look anything like it. If it looks anything like the one in this picture, but has a hard texture to it, then chances are above all it's going to be a chiton. Stomatellas are very flat and their shell only covers a small portion of their bodies. The shells look more like finger nails. The one in this picture looks like an isopod or a chiton.
On a separate note, judging by the pictures provided, it looks more like a flatworm to me. But since the op stated it was hard, then my guess is isopod. chitons generally don't climb on glass, last time i checked.