"Hello John, are you available to chat later today about Foo Product?"
Or sometimes just:
"Hello John"
I know they want something from me, but I have no idea what, how much effort it will require, how urgent it is, or whether I am even capable of helping.
A much better approach is more like this article outlines:
"Hello John. I'm looking into in a client-reported issue with Foo Product. The customer states they are unable to do X. The support ticket is <link to ticket>. I've done some investigation and think I have identified the root cause with <possible root cause>. Are you available for a brief 30 minute call this afternoon to discuss the issue as well as some potential fixes?"
That way I have some idea of context, priority, and what they are looking for from me. I can also review the details of the ticket prior to the call, which makes for a more productive conversation. Or if there is someone else who is more knowledgeable than me, I can request that we add that person as well.
If you're in the habit of starting work conversations on slack with just "Hello" - I would highly recommend that you start including additional context when initially reaching out.