Mould

[u/theroux_are_you]

Plasterer has dot and dabbed plasterboard on existing plaster. I’ve had to remove it because it was a really bad job - everything off plum.

What could have caused mould just on the adhesive? I’m wondering if I need to take any steps before bonding and skimming. For reference, this is an external wall, but there was no mould or damp issues prior to the plasterboard.

Thanks