Usually the mold/fungus come from spores still in the wood, when conditions are favorable they start to appear and grow out.
I've experienced them on some wood pieces in my tanks before too, they should not affect your shrimps (sometimes shrimps might even pick at them for food), but the mold/fungus growth can be an eyesore and might end up multiplying across the wood surface, so you should manually remove any that you find (take out the wood and scrub away the mold/fungus) to hopefully eradicate them from the tank.
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