According to SEP
There are two sets of reasons for denying that existence is a property of individuals. The first is Hume and Kant's puzzlement over what existence would add to an object. What is the difference between a red apple and a red existing apple? To be red (or even to be an apple) it must already exist, as only existing things instantiate properties.
Contra this, one can try out the sentences 'White unicorn' and a 'white existing unicorn'.
Here the adjective, or qualifier 'existing' actually adds something to the sense of the sentence; after all, for those who know that unicorns don't exist, to be told that they are adds something to their knowledge of the world.
To put the second sentence into subject-predicate form; one should say 'A white unicorn exists'...
(To which, the correct response, considering that I am a reliable informant, should run along the lines like 'My God - unicorns! really...where? Did you get some video footage...')
Notably the first sentence 'white unicorn' is just a sentence fragment, or a description; which might indicate some connection with Russell's theory of descriptions.
Why then isn't existence a predicate?