Mold under shower pan?

Lino on floor has dark spots eminating from shower pan. Testing showed it as common mold but with the amount of discoloring I'm concerned about the cause! I remodel the plumber found a broken drain about 2 feet below floor level and used a coupling of some sort to reconnect. Should we be concerned about that coupling failing after several years use? If it has failed and the ground gets wet after 2 showers daily could that be the cause of the moldy spots under the lino? We presume a plumber would be the one to call to look for cause but welcome others' suggestions. Pan itself has some discoloration as well.
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