Mould during construction

Hi,
In the middle of an extension that seems to be taking forever to build. Lots of issues with the builder I wont go into in this particular post.
So, house is about to be sealed soon, finally, should be before xmas.

The building has been unsealed for a good 2 months now due to delays with weather and roofing etc etc but primarily due to slow contractor.

Today I noticed that plywood above the joists on a new flat roof has lots of mould on it. The roof is a trocal roof but not fully sealed yet due to delays with capping and plastering. The mould is quite dense in a few patches and evident all over to a lesser extent. Builder is saying it can be cleaned off and he will dry the place out with a dehumidifier over the xmas break, and clean and treat etc.

Is this a common problem during construction phase? Is the above likely to work to rid the place of the mould, it looks very disgusting to be honest. Is there anything else that can be done at this stage?
I would truly love to fire the builder, but we are stuck at this stage if we want to have any chance to get this done.

Thanks...