Both effects of "Iron Core of Koa'ki Meiru" start a chain. Sending a "Koa'ki Meiru" monster to the GY is a cost to activate the second effect.
While "Macro Cosmos" is active, you cannot send "Iron Core of Koa'ki Meiru" to the GY to pay the cost of keeping your "Koa'ki Meiru" monster on the field. So you must either pay the reveal cost, or destroy the monster.