Amorphous. The sludge hag can move through a space as narrow as 1 inch wide without squeezing.
Amphibious. The sludge hag can breathe air and water.
Innate Spellcasting.
At will: acid splash, dancing lights, minor illusion
The hag's innate spellcasting ability is Charisma (spell save DC 12). She can innately cast the following spells, requiring no material components.
False Appearance. The sludge hag collapses into a oily pool of sludge. While motionless, the sludge hag is indistinguishable from any other oily pool of oozy drippiness.
Sludge Slap. Melee Weapon Attack: +6 to hit, reach 5 ft., one target. Hit:12 (1d8 + 8) bludgeoning damage, plus 4 (1d8) acid damage. Sludge Slap does both bludgeoning and acid damage.
You can smell a sludge hag long before you see one. A super stinky smell like rotten eggs mixed with old flower water lingers whenever a sludge hag is around. Sludge hags also tend to leave oily, dark trails of goop around the locations they inhabit. Small critters who venture too close will become covered in the oily goop and require the help of another creature to get clean.