What is this creepy little thing that I found sticking on my bathroom wall?
This is the pupal stage of a lepidopteran (i.e. a butterfly/moth). The pupa is encased in the brown silken cocoon in the centre of the 'net'. The 'net' itself is built from the setae (hairs) of the caterpillar, which it sheds prior to cocoon formation.
Here's some photos of similar formations:

http://www.richard-seaman.com/Insects/Vietnam/Highlights/index.html#Chrysalis
https://www.nhm.ac.uk/wpy/gallery/2019-the-hairnet-cocoon?tags=ed.current
https://besgroup.org/2015/11/05/a-parasitic-fly-emerged-from-the-cocoon-of-a-footman-moth/