Inside the nest: cocoons inside 'cotton fluff"

There have been half a dozen reports of cotton fluff inside the nesting tunnels.  Here is one I found myself.  Most of the fluff is just that, but two cocoon type structures were found in the center row.  If someone knows what this is please let us know.

A nesting tray with 6 routered channels containing mason bee cocoons,
and cotton type fluff in two of the channels.  

Here I have lifted some of the fluff out to show how it neatly fits into the channel.

Two cocoons were found inside this fluffy material.
You can see the end cap directly above where the cocoon is held in the photo.
 The end cap is made of several layers of mud and is thicker than the usual mason bee end cap.

For comparison, this appears like a spider web,
 which either contains young spiders or an adult spider.