It's not technically wrong, no. Nobody will accuse you of having bad grammar. But every good writing style manual warns against using it. Because the active voice is clear, bold, and more concise. There are only a few situations where writers should prefer the passive voice.
Many "good writing style manual[s]" are kind of full of it.
An interesting bit from the wiki(1) on passive voice: "For example, despite Orwell's advice to avoid the passive, his Politics and the English Language (1946) employs passive voice for about 20 percent of its constructions."
As with anything, it's important to be aware of what you're doing. But back when I tutored people in writing, hard-and-fast rules like this resulted in awkward, contorted writing by scared students. For example, I once knew a very smart person who avoided predicate adjectives at all costs. I don't really blame him for confusing passive voice and predicate adjective. I even found cases where the Hemingway app confused the two. But it was too bad that he had been so thoroughly brainwashed against the passive. Though I kind of admired how he managed to avoid, for years of his life, what I think is an indispensable construction.
I haven't had the experience of tutoring anyone in writing, but I'd like to. And I agree, that you can't place too much importance in style manuals ... they can give contradictory or just plain stupid advice. But good style manuals do more than set rules - they explain why the rules are there, and even highlight exceptions where breaking the rule is appropriate. They are indispensable for improvising your writing, provided you don't treat them like rulebooks. Read a few, then form your own opinion.
Now, unlike a style manual, Hemingway doesn't explain the reasoning behind rules, or permit breaking the rules in the right context. It's limiting, and won't produce beautiful writing. But I suspect it will have use for non-writers. I'm talking about people who write imcomprehensibly, and don't have any incentive to master the art of writing. Doctors, lawyers, and business executives come to mind. In my experience, people in these careers write like shit and don't have time to improve their writing. By adhering to these rules, they can improve their writing significantly without much effort. The result will still be somewhat awkward and contorted, but far less than what they would write otherwise (marketspeak / legalese).
As my sibling commenter said: there is no situation where the writer should artificially contort the sentence just to avoid using the passive voice.
The examples in the Hemmingway app are great examples of this. There is nothing wrong with the phrase "Phrases in green have been marked...". It is perfectly descriptive and reads quite naturally.
The passive voice is correct grammar. It is 100% properly constructed English. It's a matter of style. And while one should be aware of and able to manipulate their style, that doesn't mean there is anything wrong with any particular style.
Even long, frequently forking sentences are not wrong. They are just a form of style. Style is about conveying mood, and cannot be prohibited any more than the choice of words in the sentence can be prohibited.
