What's on Denethor's face?

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

Chicken has blood.
And also, I'm not sure Denethor eats tomatoes and grapes. It can be any imaginary vegetable o frutta 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 te 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 pomodoro succo, succo di frutta o wine, but it is the right colour to be blood

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