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.