"Turritella" slabs with white discoloration

[tangers40]

I have several pieces of "turritella agate" (really Elimia I guess?) that I found in northwestern Colorado this summer.  One of the large pieces I am wanting to display outside in my yard, but when I pulled it out of the dirt, I noticed most of the best fossils it contains are covered with a white discoloration.  The entire back side of the piece, which was in the dirt, is also covered with the same material.  The original 'matrix' that I"m wanting to display is normally a dark grey/black color.

 

I thought it was just dirt/mud, but it does not come off with simple soap and scrubbing.  Can anyone tell me what it might actually be, and if I would be able to remove it somehow?  I know the pictures are terrible, if needed I can try to take better ones.