Hi I just got my fist tank this weekend and it came with everything including fish.  The lady I purchased it from had sea shells mixed in the gravel.  One shell had never opened, and today it opened and there is a mucus white stuff in it, and my tank is a mess today.  Something has burrowed holes in my gravel.  can anyone tell me what it is?  is it from the shell?  I have pictures of it, but haven't figured out how to post on here.

I can't say anything for sure but I suspect that the "seashells" you refer to might simply be aquarium snails that were not removed by the previous owner, and simply allowed to die and dry out. When you added water the dead snails may have started to rot, causing bacterial and probably also fungal contamination of your tank. This can happen if a fish or other animal dies in your tank in a place where you don't notice it. It can also happen if you put more food in the tank than the fish and other inhabitants can eat.