How to say hi to someone without it being awkward
Awkwardness usually comes from a speech, not from a hello. Keep it to one line, then leave room for them to answer or leave.
In the app, stopping someone is the hello. Say what you noticed, not a verdict on their looks. “That coat is excellent” is plenty. “Want to walk as far as the square?” gives them a size for the conversation.
On a real street, the same shape works. One sentence. Eye contact. A pause. If they look away, smile once and go. A no that is allowed to be quiet is kinder than a follow-up.
You don’t need a story about yourself yet. If they stay, you’ll get to it. If they don’t, you both still have the rest of the walk.