It's a wasp nest which is far too common in older lights, they may of been built strong, but they didn't keep wasps and birds out. Luckily all new fixtures are designed so birds and insects can't get in.
The newer fixtures have a shroud around the slipfitter area, but the lenses are still cheesey. The older lights used glass or thick plastic. The newer lenses like the M-250R2 and OVZ acrylic lenses are so thin. I see birds' nest way too often here. You can always tell where a birds' nest was because here when a nest is built, a GE poly lens is put in. When lights are installed new though, they use acrylic.