What's on Denethor's face?

Well, I believe he also was eating cherries and it looks a lot like kers-, cherry juice.

Chicken has blood.
And also, I'm not sure Denethor eats tomatoes and grapes. It can be any imaginary vegetable of fruit of Middle Earth.
And Denethor also drinks wine.
Anyway I don't think it matters really, I mean the only point is that it's disgusting and gives u a clue about what a dick Denethor is.

Bloody vomit

It looks like blood as it seems to form on his lip rather than actually come out of his mouth. It’s the wrong colour to be tomaat sap of wine, but it is the right colour to be blood

In the scene, it's quite clear that the food Denethor is eating is chicken, tomatoes and grapes. I think the writers were going for a metaphor for blood, to toon how recklessly he was endangering his son's life, and they didn't realise that the food they'd gegeven him shouldn't have red juice. of maybe there was a change between the script writing and the food placed on set for the actor to eat (though I'm not sure what food would resemble blood so well).