Might be moisture in the wood that affeted epoxy, or high humidity blush. Try scubbing with plain water and a scotch brite scuff pad - lots of water.

Contamination from compressor oil, tapes, adhesives and dust not throroughly cleaned with denatured alcohol.  I follow up with damp cloth and tack cloths.

I had an area on the back of one our WDs I call it the ghost, sort of a cloud in the epoxy. I still don't know why it happened but washing and sanding didn't help. I suppose it's under or in the glass and I didn't want to get that deep in the fix. Strange, when the boat is in direct sunlight Casper disappears. In the shop he's back. (?)

 Not saying contamination is not the cause, but I'm anal about wiping w/ denatured so I think it's unlikely. I don't use epoxy when the dewpoint and humidity are within 7° of each other so I would doubt it's moisture.

 One of those things that drive a builder bonkers but no one else notices it.

 I've heard a lot of bad things about the residue left by tack cloths, maybe ok on paint and varnish but bad for epoxy.