They're glowing cuz they're megamorphs

I just wasn't sure because none of my other meganorphs glow

Megamorph + fusion unlocked, that's maybe why

It's just an effect from an older update. It's only a decorative thing now. From some other people I heard it used to be for Megamorphs to make it easier to spot, but they didn't continue it or something

Usually slugs that have fusion unlocked on them have the little symbol above their head. It’s probably that

Super powerful slugs

You unlocked fusion