Corner of MacBook Pro (M2) lighter after water spill

—Is it likely to go away in time as things dry out?

Based on history, probably not, but one can hope.

—Or should I expect further issues?

Again, based on history, probably yes.

Liquid damage has bad habit of causing corrosion that gets progressively worse over time. Corrosion is far more likely with acidic liquids like tea, coffee, or carbonated drinks than with plain water.

If I take a screenshot, the lighter area doesn't appear on the image.

That observation make this scenario:

Another possibility is that some liquid has somehow got in the screen's laminate...

far more likely.

If this is a Retina Macbook Pro, expect to be quoted a cost of US$500-650 for a display service. The components of the display cannot be separately replaced of repaired. The damage you see will require a new display assembly to properly fix. 💰

⚠️ Before spending that kind of money, you REALLY need a full professional eval to see if there is internal damage to the logic board and other chassis-mounted components.

Otherwise, you can spend over $500 on a new display only have another expensive failure later as the acidic liquid does it dirty work.

If the eval finds damage to any cassis-mounted components , you will be better off getting a new computer.

You should schedule time with an Apple Store (or Apple Authorized Service Provider) to assess the damage and replacement cost. This is an acid-based time bomb and it will continue to get worse…