Heres a couple of foto's of visitors i have in my tank, I can only get a foto of them "rear on" whilst stuck to the glass, as when not stuck to the glass they fly around the tank in the current, and under their own steam.
side on, they look like a shuttlecock, they are around 3 - 6mm long, and swim by convulsing. My guess is a jellyfish, based purely on how they move, but that really is a guess.
yup thats what I have. Pretty sure they're not glass anenome. They dont look like any of the pictures I've seen of aptasia and they act like it either. I hadn't noticed the ones on the glass where the same as the tiny jellyfish I very occasionally see.
Going to do some googling on these Claudonema jellyfish now.
//edit// yup that set me on the right track. I believe they're Hydroid Jellyfish or Staurocladia oahuensis (mine are at least, yours are a little bigger.)