Yes, that is mold.
Yes it is something to be concerned about. The water is likely rotting away the wood behind the wall, and the subfloor.
Lots of possible sources. If that is not a basement, most likely either you are on a slab and the dirt outside is too high, or a leak in the roof and water runs down the rafters and into the wall.
You can clean up the mold yourself. Any hardware store will have Concrobium. Comet might be good at removing the stains from the carpet.
But you have to find and fix the source of the water.
I have no idea how long its been leaking, or how much water has entered, so my guess of damage is of no value.
You may wish to climb into the attic and see if you can find signs of water entry. Might not be obvious.
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