Why is bubble present, in this redox reaction?

[Nope]

Homework Statement

In the lab, i saw that when Mg(s) attach with Fe(s) nail in a yellow solution( probably indicator), after one day, the brown substance and bubble are formed.
My net ionic equation is
SOA: O_2(g) +2H₂O_(l) +4e -->4OH^-
SRA: Mg_(s) --> Mg²⁺ +2e
NIE: O_2(g) +2H₂O_(l) +2Mg_(s)-->4OH^- +2Mg²⁺
Can anyone explain why is bubble formed? and what is the brown substance?(is it MgO or Mg(OH)₂, or Fe something??)
Thanks

Homework Equations

The Attempt at a Solution

 

[flatmaster]

I immagine it might simply be iron oxide... rust