I like the idea a lot. I think it's quite flavorful. There actually was a stone golem in return to the temple of elemental evil that started as a mosaic on the floor.

I find the shard spray and the treadeasy effects a little too simplistic. I would have done the following:

Give the creature Improved Grab, and a +4 bonus to grapple from the template.

Then give the creature a Trap the Soul effect. Essentially, once the creature grapples with an opponent, it will try to move the grapple until it is next to the mosaic. Once standing on or next to the mosaic, the creature will attempt to pin. If it succeeds on a pin, the grappled opponent must succeed on a Will save (DC creature HD/2 + Con, maybe) or appear in the mosaic, body and soul, just like Trap the Soul. Each such mosaic has a particular tile (maybe a gem) that can be shattered to free all imprisoned souls.

Just my thoughts.