It is unlikely that you will have rising damp in a brand new home. All new homes have a damp proof course (DPC) bedded in the cavity walls, which will stop damp rising from the ground through porous brickwork. However, this can be bridged if the external ground level is above the DPC or the cavity is filled above DPC level with mortar or debris, or if the DPC is covered by render. The DPC should be at least 150mm above finished ground level.
It is more likely that your problem is caused by
CONDENSATION, which can be improved by better ventilation/air circulation and heating the room.
Treat the mould with a diluted bleach solution, and open windows on fine days. It can take up to 9 months for a brand new home to fully dry out.
It may be caused by moisture penetrating the outside wall, due to leaking downpipe or guttering combined with the cavity being bridged with debris. Contact your house builder if the problem persists.