It could be what is called a "floater" A small piece of the...

It could be what is called a "floater"
A small piece of the jelly like material that fills the eye may have detached from the retina.
This occurs quite often as people age.
I got a number of these after being hit in the eye during martial arts training.
They often fade or disappear after some time.
There is plenty of information on the net about them.