If it’s not a type of fungus then it is a type of bacteria growth.
The wood is in it’s early stages of breaking down, in theory the shrimp should eat it overtime. Give it about two weeks, but keep an eye out on your ammonia and nitrates.
Luckily you have a Java Fern which will suck up any of the CO2 that’ll be released.
If you’re worried about it add in Catana leaves to increase acidity that’ll help against fungus. If still worried take the wood out and rinse it with vinegar to kill what is growing.
Warning it might grow more later since the stick is essentially early stages of breaking down.
Niks-a-dick 
If it’s not a type of fungus then it is a type of bacteria growth.
The wood is in it’s early stages of breaking down, in theory the shrimp should eat it overtime. Give it about two weeks, but keep an eye out on your ammonia and nitrates.
Luckily you have a Java Fern which will suck up any of the CO2 that’ll be released.
If you’re worried about it add in Catana leaves to increase acidity that’ll help against fungus. If still worried take the wood out and rinse it with vinegar to kill what is growing.
Warning it might grow more later since the stick is essentially early stages of breaking down.