What is this in my air duct
It's &quot;trash&quot;.   This is exactly why I don't like flexible air ducts lines and I only use them if I absolutely have to.
Couple of things:

this fell between connections.  Connections should be sealed/taped.   That isn't a good sign.
anything in the way of your line will decrease effeciency.
might just be the picture... but your line looks dirty.   I don't often say that but it needs a cleaning.   This is another reason why flex lines aren't as good.  They are dust collectors.