what's that on my gpu heatsink

[Rishi_Adhikari]

[KeyringHardhat]

Could just be oxidised aluminium, at least that's what it looks like in these images. You should disassemble the card before any kind of cleaning, and clean it with some weak acid like white vinegar. Rinse off with isopropanol and reassemble when dry. Great excuse to replace thermal paste too (Be careful around the thermal pads, you likely don't have replacements, take care of them).  

[8tg]

Tarnishing and corrosion, it happens over time depending on the environment. A lot of post mining gpus show this pretty heavily because they’re kept in rarely humidity controlled warehouses. My 1080 mini I got recently has this pretty bad.

it doesn’t really affect much, if you mind the aesthetics you can try hitting it with TarnX but it’s the metal plating that has corroded, so it will never look perfect.

[PeachyUwUSenpai]

its mostly likly the metals in the plating reacting due to heat and age assuming youve had this card for a while. cleaning it will most likley just lead to more reactions so its best to just leave it alone.

[jaslion]

Happens over time but can also just be skin oil from touching that reacted eith the metal over time causing more staining. Ive seen this happen often. Entirely harmless.