Copyright: milkext
Milkext is a "white liquid GIF animation tool" hosted at:
https://mygamestudio.x.fc2.com/milkext/
The site currently seems to be region-locked, but still functions from within Japan (as tested with a VPN).
Animations outputted from the tool have a recognizable pattern and do not preserve one color palette per frame, resulting in a distinctive shimmering effect due to the 256 color limit for GIFs.