Iron bacteria convert dissolved iron (ferrous iron(II)) (Fe2+) into ferric (iron(III)) (Fe3+) by oxidizing it with oxygen to create an insoluble reddish-brown gelatinous slime.
The process releases energy, which is useful to the bacteria.
So, you need ferrous iron.
Stainless steel contains elemental iron that is not oxidized.
Only if the iron is oxidizing (rusting) will it be susceptible to iron bacteria.
So, the primary problem is due to the defects in the stainless steel causing it to rust.
Because the steel is under water, it can form many different compounds.
Maybe bacteria is involved and maybe it is not, but the primary issue is defective stainless steel as evidenced by the many other posts about rusty bands.
As the steel is heated and machined, the molecular structure is changed and the steel becomes susceptible to corrosion.
Pool water that is properly treated with chlorine should kill any bacteria.