GrowMask can now be used with negative numbers to erode it.

comfy_extras/nodes_mask.py

 import numpy as np
-from scipy.ndimage import grey_dilation
+import scipy.ndimage
 import torch
 import comfy.utils
 
@@ -311,7 +311,7 @@ class GrowMask:
         return {
             "required": {
                 "mask": ("MASK",),
-                "expand": ("INT", {"default": 0, "min": 0, "max": MAX_RESOLUTION, "step": 1}),
+                "expand": ("INT", {"default": 0, "min": -MAX_RESOLUTION, "max": MAX_RESOLUTION, "step": 1}),
                 "tapered_corners": ("BOOLEAN", {"default": True}),
             },
         }
@@ -328,8 +328,11 @@ class GrowMask:
                            [1, 1, 1],
                            [c, 1, c]])
         output = mask.numpy().copy()
+        while expand < 0:
+            output = scipy.ndimage.grey_erosion(output, footprint=kernel)
+            expand += 1
         while expand > 0:
-            output = grey_dilation(output, footprint=kernel)
+            output = scipy.ndimage.grey_dilation(output, footprint=kernel)
             expand -= 1
         output = torch.from_numpy(output)
         return (output,)