It may really be cement. The guy who installed the tile may have used the drain to dump the cleanup water or was really sloppy when using the thinset or drypack. If the pipes are pvc, then you could try to break it loose. Another possibility is if a sanded mastic was used instead of a real cemetaceous (sp?) product. The mastic re emulsifies, gets washed out, and can gradually undermine the tile.