First of all, why don't you have the seller remove it? If you break any pieces.....which you might well do, what are you gonna do?

What is the display board made of? Could it be soaked in water to make it disintegrate?

It would take you days, but I would think that you could slice each mosaic piece from the board using a scalpel or another thin blade tool. Silicone is a rubbery material when cured and I think the easiest way to free something from it is to slice it free.

Don't do it with yourself if you haven't any experience of it. It can break your tiles. Take a professional service for it and they can do this for you.

One alternative might be to buy a Fein saw or a generic Fein saw like a Dremel or Rockwell Sonicrafter then use that to cut the tile off the backer board.