Just speak your mind or even better: just send an email.
IM is really great for sexting but it’s not great for anything professional IMO. Transcripts aren’t always saved and you can’t trust the transcript from a lot of clients (I had one person who would often edit messages to completely change their meaning after sending them, I had to make a habit of screenshooting IM conversations.) If you really need to have a conversation and can’t talk to me in person you can probably get my phone number.
The people who do the "Hi" thing are savages and need handled appropriately.
Options:
1. Ignore their message until they've fully communicated their initial ask. it is not your responsibility to engage to "Hi".
2. Sporadically type something in to the chat box but never actually send the message. This gives the other person the "bubbles" but they have to keep waiting for a message that will never otherwise appear ; )
I'm sounding like a zealot here, but this is why I like Zulip. It has multi-line compose by default and it encourages you to write messages like you write emails, ie long-form.
Is slack etiquette different from that of Skype?
I prefer to greet and then send in my message(in one or many chunks depending on complexity/feedback) after that.
The greeting is irrelevant because truly neither party of the message cares. That's okay too. You have a question, I probably have an answer so just ask your question.
The absolute most respectful thing you can do to the person you are messaging is to send as much info as possible in the original message and then let them respond. If you do need to engage first with something like "do you have a moment for a question?" then still write your question first so you can copy/paste it immediately.
In my experience this is usually done by someone making a request outside the normal chain of command and therefore trying to signal politeness. And it backfires.
Gather your thoughts, then write everything - the context, what you need from me, everything - in a single message.