Can I cancel a signed apartment lease in Greece if the landlord hid mold and refuses repairs?

Yes if there are serious problems for which the landlord is responsible and for which u were not clearly informed when u rented, u can cancel the lease and ask the deposit (guarantee).

Generally, serious undisclosed defects that make the property unhealthy or uninhabitable can allow the tenant to terminate the lease early without penalty, reclaiming the deposit (if undamaged otherwise), further claims also for possible moving costs or other compensation may also apply. However, at times success depends on proving the defect's severity, so outcomes vary case-by-case. Finally, for the proper stategy and evaluation of the case a review of the relevant documentation is needed.