Mould Patches on the Microcement
Yellow, orange or black patches that appear on the microcement surface over time are most likely caused by mould.
Most of the time this fixed with a good scrub & regular cleaning, see the guide below for dealing with more stubborn mould patches.
Does Microcement get Mouldy?
Patches of orange, yellow or black that appear slowly over time are most likely to be caused by mould on the surface.
Microcement is generally very good at resisting mould, because there’s no grout lines, so there’s not a lot of opportunity for it to take hold.
Regular cleaning should prevent mould from appearing & keep it under control if it does.
If you do discover mould patches on the surface, then try scrubbing cleaning product into the mould with a white scouring sponge. To see if that starts shifting it.
If that doesn’t work, try mould remover.
And as an absolute last resort, you can use a 220 or 120 grit sanding disc to sand back the surface to remove the mould and then apply another coat of sealer, to replace any that you have sanded off.