Did you have a gas bubble in your eye to replace the removed vitreous?
If you had a bubble, then you may be seeing the bubble edge. As the eye refills with natural fluids (aqueous) and the bubble shrinks, you will see the bubble edges, and eventually the whole bubble outline. The gas bubble will appear in the bottom of your eye, since the image in your eye is inverted. What you will see at the top of your vision first is the bottom of the bubble.
If the surgeon also used PFO (perfluorooctanate) liquid (an inert, heavier than water chemical compound) in your eye during surgery to "flatten" the retina against the interior of the eye, then there may also be some residual PFO present. It would be in a separate bubble from the gas. Since PFO sinks, you may be able to see it at the top of your vision. If it's "a lot", most of it may be removed in the doctor's office, but a little may remain for a long time.
Small floaters could be normal blood cells or bits of tissue floating around in the liquid in the eye as a result of the surgery or the healing process. Noting any changes in size or quantity or volume is important information for your doctor. If there are significant changes, tell your doctor. Lots of new, big or dark floaters would be a concern.
I saw all these things when I had cryo+bubble (twice), vitrectomy+laser+PFO+bubble. It takes a while to get used to seeing these strange things happening inside your eye. If you look up a diagram of the eye, and imagine where the bubbles of gas/liquid are, you can get an idea of why you are seeing what you do.
Hang in there, and good luck for your recovery!
A retinal detachment repair without a vitrectomy does not removed the floaters. I assume this opaque band at the top is not residual inferior retinal detachment.
This is not a sign of cataract formation.