Brown algae is usually caused by little light in a tank.
I have to agree with Sena on what it could be.
However, the fungus doesn't have to come from anywhere. Things like fungus and bacteria that cause fish illness is already found naturally in the water. Normally it's in small amounts that the slime coat protects against, but something could have weakened the slime coat to let the fungus set in.
If it looks like a crack it can very well be slime coat/tissue setting in.
If it starts spreading it's a fungus, if it stays in just that one spot it's most likely a cut that's healing. Take the time to recheck your tank decor.
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