looking to the reverse-engineered client.dll, it seems that the only message param is sprite index.
It means, that when you want to hide it, you need to send 0 as a param.
But when you need to show it, you need to send sprite (sprites/shadow_circle.spr) index.
Also it seems I was mistaken at the beginning. You can safely send it to the one player, instead of sending to everyone.