Hi Julie i would recommend that you strip the floor back to the concrete.
I know this may seem like a lot of hard work but better to be safe than sorry.
It is not really adviseable to tile over floors at the best of times let alone when mosaics are involved as the individual mosaics do not have that much adhiesive holding each tile down , that is why certain tiles are blown at the min.
So like i said remove the tiles and charge for this .The floor might then need a thin layer of self leveling over it .
Prime the floor with a suitable acrylic primer then your set for tiling.
Hope this helps
