Is it dry, wart like growth, or does it bleed or drain? Is it still growing since you first posted?
If it truly is wart-like and doesn't bother the bird, you might just leave it alone. Or if you just don't like the look of it, you can try the freeze-off wart treatments available in most pharmacies or pharmacy departments. That said, chickens' combs are very susceptible to frostbite, so it could be risky (or at the very least, tricky to just treat the growth).
If it is bleeding, leaking fluid or pus, then my advice is to treat it with antiseptic dressing (I use the antiseptic spray dressing Wound Kote) and see if it gets any better.
Hope this helps!
-Sara