Funny that I read this today, because I noticed just yesterday that I have the same brown spots on my bath bomb cupcakes in between the icing and the bath bomb. Honestly, I don't know what it is. But I think it has to do with the FO. Have you used the same FO in anything else? Have you noticed any discoloration with it? I also thought maybe it could be any retained moisture from the bomb coming out the top? Or maybe the icing has been "sweating" and reacting with the bath bomb? Is your soap icing the Royal Icing or is it whipped soap? If it's soap, did you use melt and pour? Maybe you heated the melt and pour too hot and it started to sweat?