Actually, it isn’t. What you see is the glow layer rendering on top of the object. Also the glow is applied to the entire mesh. As briefly exposed in my last post, the issue with the glow layer is that it cannot be worked through the material (i.e. an opacity texture). It’s a (fairly old) post-process fx that renders on top of the scene. The only way to create a mask on it is through the rendering of two cameras.
However, the glowLayer has a property similar to lights. You can include or exclude meshes from the glow layer. Now, for this to work on your snail, you would need to not merge everything together (though, I didn’t check your object so if you have submeshes then you could select through the submesh). Else, I believe the easiest would be to merge all of your non-glowing meshes and merge and parent to your snail all parts you want to glow. This way you can include only the glowing mesh/submesh(es) in the glow layer.
There is still another issue though with the glow layer (sorry, not the best process for learning BJS 
) The issue is that when I say ‘mask on the layer’, it’s actually a bit more complex than a simple mask. In this aspect, the highlightLayer would be easier to handle but it is also even more performance consuming than the glow layer.
In order to mask the glow layer from the objects where you don’t want to see the glow showing through, you actually need to add them to the glow layer. Yeah, I know, it’s weird. The first two things to understand here is that 1) The glow layer is a post fx (layer) that always renders on top 2) The glow layer uses an emissiveColor, with alpha and intensity to create the glow.
The idea is to set a custom emissiveColor for the glow layer on these masking objects. This custom emissiveColor comes with an alpha of 0(zero). That’s the important part. So, read here that when the glow layer sets the emissiveColor on an object tagged to use an alpha of 0 then, since there is no alpha, there’s also no glow. This is the method I exposed in my rev of your PG below.
And then, there’s still another solution (there nearly always is 
). You could create a node material for your glow layer.
Here’s an example/PG for you and if you want to tackle the subject in the forum, there are a number of interesting posts around this topic:
I hope this helps and I’m also happy to hear that you seem to enjoy your experience with BJS and
We love that spirit here and we’ll support you the best we can.
Meanwhile, have a great day 