When you say "floating on the surface attached to the glass" where exactly? Totally submerged, partially submerged? Although I am a newbie to saltwater tanks, I am a biologist and have to say that newreefers could be right. The pics a bit blurry, do they have a hard casing? What's inside? I say take one and open it and inspect the inhabitant - you'll be able to tell cleary if it's a terrestrial or marine creature pretty easily. If you have ANY openings at the top of you tank an insect wuld definitly take advantage of the moist, warm climate inside the top of your tank to hatch some babies.