There's plenty of talk on the evils of starting a conversation on Slack/IRC/whatever with "hey". Just say whatever you're going to say, don't waste my time with Hey.
I'd like to come out with a contrasting opinion. Should you start a conversation on Slack with "Hey"? It depends.
You see different people are different, you can examine people through the lens of True Colours, MBTI, whatever. When you're pitching a new piece of work to get done some people will respond best if you attach some emotion to it, others if you back the need with hard data, others if you approach them personally. None of them are wrong, they're just different.
It's the same thing when it comes to "hey". If I need something from Gemma, I'd probably open with "hey gemma could you check my code in PR #582, I'm trying to ship that before I head out", if I need something from Caroline it would go more like "hey". Gemma wants to get to it, Caroline would like to open up the conversation first.
I don't think either of them are wrong! They're both right for themselves. As a manager it's my job to work with my team as best I can, so this is one of the things I'm trying to learn.