Does it glow orange with the Atinic light on?
Orange looking coralline at that stage could be any color, i have the blood red type CA in my tank and the new growth portions always glow hot Orange in the atinic light.
I do'nt know about metal and i'm suspicious as to what it really is, that rock looks too new i'd be suprised if it were coralline algae but it could be.
You'll have to just watch it.
I used that dry rock for my tank too and i noticed little green balls first then a massive diatom bloom. i didn't get any coralline until i managed the Calcium, ALK and Mag in the tank, about 6 months from start up.
I hate it...my sump is ruined with it, can't see anything...lol