What is this black stuff appearing on the walls of my hostel’s bathroom? It’s almost in every corner as well. Should I be worried?

https://ift.tt/2PEU12i via /r/whatisthisthing https://ift.tt/2AfbKYK
What is this black stuff appearing on the walls of my hostel’s bathroom? It’s almost in every corner as well. Should I be worried?
Reviewed by M. Amaar Tahir
on
12:30 AM
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