The story
- Proto-Indo-European*sley-
- Proto-Germanic*slīmą
- Old Englishslīm
- Englishslime
- Proto-Indo-European*bʰel-
- Proto-Germanic*balluz
- Old English*beall
- Middle Englishbal
- Englishball
- English todayslimeball
From slime + ball. Sense 1 (“a round lump made up of or coated with slime or a slime-like substance”) is a calque of German Schleimkugel, from Schleim (“mucus, slime”) + Kugel (“ball”), while sense 1.1 (“a mucus-coated lump containing the cercariae of a liver fluke”) may be a calque of German Schleimball (from Schleim (“slime”) + Ball (“ball”)).
Etymology from Wiktionary, CC BY-SA 4.0.