Roller wheels on sliding door + stubborn black mold in concrete?

[WatsonClean]

Hi. Thankyou for taking the time to read.

So I thought I'd do 2 separate jobs in 1 post if that's ok.

There are 4 concrete sections in an alley way. 2 sections have this black mold or pigment within it that I can't seem to even discolor let alone eradicate. I've even gone to the extent of using pure hypo multiple times with no change on colour. And the use of a 4k psi jet wash with no change in colour.... Any tips or ideas on what this even is? Pictures attached:

Also I've never done maintenance on sliding doors and would like some advice on what tools/products and lubricants to use in regards to the initial clean and then maintenance for the customer 2 X per year. (It's the rollers that need the maintenance)

Im fairly new to posting here and haven't contributed to the community myself and am asking for help..and I genuinely very much appreciate any input. Thankyou again for taking the time to read.

 

[ched999uk]

The rollers do look a little corroded but I would be tempted to use silicon lubricant on them. Don't use WD40 as it's not a lubricant and actually contains solvents which might not be good for upvc.

Sorry no idea about the concrete.

 

[Pjj]

That black on the path isn’t in the cement it’s self is it ? Neat hypo would kill anything if it’s mould / bacterial growth

 

[WatsonClean]

The rollers do look a little corroded but I would be tempted to use silicon lubricant on them. Don't use WD40 as it's not a lubricant and actually contains solvents which might not be good for upvc.

Sorry no idea about the concrete.

Thankyou. Noted! It's actually a sprayed aluminium. Would a scouring pad be suitable with a dash of oil to get the rust off initially? I don't want to scratch the ally. And on the insides it's a dark grey colour so could easily scratch the paintwork FFS .

 

[WatsonClean]

That black on the path isn’t in the cement it’s self is it ? Neat hypo would kill anything if it’s mould / bacterial growth

That's what I'm thinking. But why! Could the builders of spilt something in the mix? Because there was green algae over the top..took away the algae and now have this black I can't shift with anything.
I don't mind telling the customer I can't get it out but I would just like to tell him why I can't get it out. And what I think it is. Thank you mate