I would think that the power led would be on if there’s power going through the motherboard.
Are you referring to the power led or the disk usage led? I think on my nuc the only other led is the disk usage one, which looks like a soda can. The power led is always on, but doesn’t indicate standby or operating as far as I’ve seen.
That looks like “seasoning” you’d get on baking sheets and cast iron. Seasoning is polymerized oils that end up having a high temperature breakdown point, quite a bit higher than the original oil. It’s not an issue, and if you’re willing to put up with it, might actually make it easier to clean since it has non-stick properties.
Otherwise, you want to use a basic (as in acid-base) cleaning agent, ammonia, oven cleaner, etc. should work; bleach might not be a strong enough base. Oven cleaner is made for cleaning this type of thing, but it’s one of those cleaning agents where the precautions are absolutely required, not just a company liability thing because idiots.
In my opinion, if soap and water and scrubby pad don’t remove it, it’s not worth further effort.