Mulchmother (click for map image) spawns among the tasselvines in the east-central part of the island (PH "a twisted tasselvine").
Map points for "brotherisland.txt" map files (for all Brother Island named map points, see this quest entry):
It casts "Coastal Curse" (DOT for 23,000 dmg/tick) every 50 seconds (38 corruption counters).