A generated clock should reference the clock that is propagating in its direct fanin, although the create_generated_clock command lets you specify any reference clock. This DRC warning is reporting that the generated clock is associated to a master clock defined farther upstream than the incoming master clock. In this situation, timing analysis can be more pessimistic and apply additional clock uncertainty on the paths between the master clock and the generated clock. This can lead to slightly more difficult timing closure. Is is recommended to associate the generated clock to the master clock source pin that the generated clock is derived.