I would need a much more visible masking color.
Attached a screenshot of Polygroups, where is partially a mask on it.
Do you see it? not? me not, too…
Example: Photoshop has by default a strong red, and depending on your object you can customize this.
In Zbrush there is a huge iColor palette, but the most important is missing: Selection highlighting and mask colors.
Please consider for future versions.
If you can’t see your mask, turn the Ambient up on your selected material (Materials Menu > Modifiers > Ambient)
Though it would be much easier to use the Select Lasso to isolate the areas with these unneeded spikes, and using the grow selection(ctrl Shift + X) and shrink selection(ctrl shift + S), or find them in Tool Menu > Visibility.
Then polygroup when isolated (ctrl + W), invert your selection and delete hidden and close holes. Tool Menu > Geometry > Modify Topology > Delete hidden / Close holes.
Having a coloured mask could become awkward when polypainting, it works nice the way it is.
Hope this helps your situation ~ Cheers