I don’t know what it is but I have to stare at it all day 
There’s a label that says the wood has been treated with copper azole wood preservative but I’m not sure what this stain is, how do I clean it off without damaging it?
OP, I actually do woodworking as a hobby. A couple posters are correct that this is the copper treatment to make it not rot. Sometimes in the pressure chambers the chemicals concentrate in a spot, or the wood can absorb it more in one area. You can sand it off, but please wear a mask and eye protection every time you create sawdust. Even without the chemicals in it, sawdust is horrible for your lungs, and the smallest fleck in your eye can ruin your entire day. A less hazardous way to get this off is with a hand plane, but it takes a bit of experience to use it and blend it all together well.
It’s not a fungus, but copper-treated wood like this can be poisonous, so sanding it down might not be the best option. You really do not want to breathe in that dust.
I’d reccomend painting over it or just hang some art or a poster over it.
I’m no wood expert (well, this kind) but I’d say that’s a fungus growing, possibly some water damage. I would assume that plank got missed for treatment or it wore off over time. So the good news is you don’t need to worry about damaging it, because it’s already damaged. You can try sanding it off and reapplying stain and sealer, but it’s probably rotting inside the wood, so I’d say you’re better off replacing the whole plank.
Edit: I agree with a reply, most likely the copper treatment leaking
This isn’t a fungus, this is the copper treatment, that soaked into the sap hole, bled out and oxidized into the blue green color. It’s because of the wood treatment.
I think that’s what it is, is there an way to remove it or lighten it?
Sand it
Yeah, that makes sense. Like I said I’m no expert. The wood splitting on the right side would indicate other issues with the plank as well, no?
It just got installed by property management, it’s the only problem board and it got unfortunately placed right in front of our window lol
If it’s not yours then fuck it. Sand away, sister.
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Your first step then should be complaining to them about how they installed fucked up wood and caused an eye sore. It’s their property, they should fix it
That doesn’t mean they will but it’s worth a shot first over paying for/doing anything to fix it yourself
So the good news if you don’t need to worry about damaging it, because it’s already damaged.
“What’s the worst I can do, it’s already fucked up” is such a good feeling lol
Flip the board over of course.
Can you put a poster or stickers over it?
I could make a map out of that
If you decide to sand it, I’d check if the stuff used in the wood is toxic. Some of them are very harmful and you would need a good enough mask, skin protection and such to sand it.
I know nothing about what is used as preservative where you are, but at least here there are toxic versions that you can’t even saw without protection.
Idk what you consider damage but I guarantee a power sander will have it gone in 15 minutes tops.
Fence posts are like $6. Do you have a drill?
