Asklemmy

I recognise it as being used by Pink Floyd, but I don't recognise it as a symbol of Pink Floyd fandom — that's more commonly the prism, or any of the album covers.

Could just be a symbol they thought looked cool without any real meaning behind it. Something like that, I'd just ask. And play dumb in case they're dangerous. Like "isn't that from Pink Floyd or something?" If they shy away from the question or say something like "nah it means something else" it's probably exactly what you think it is. Then again, if it is Pink Floyd, expect a lecture. Just tell them A Momentary Lapse of Reason was better than Dark Side of the Moon and see if they write you off as crazy rather than try to correct you. (I do personally think Momentary Lapse was the better album, but I'm not about to tell a Pink Floyd fan that. The hot take I will share with one is that the one with the stupidly long title on Ummagumma (they'll tell you the whole name) is one of their most ambitious tracks. That, I think, might at least get some respect. (The title is something like "Several Species of Small Furry Animals Grooving With a Pict." But I think there's more to it.)

The crossed hammer symbol has been used by neo-nazi groups, yes.

Hammerskins also use crossed hammers: https://en.m.wikipedia.org/wiki/Hammerskins

Pink Floyd fan here. I would not wear this because even in the pink Floyd lore its a representation of a fascist group. Even in the best case its still a dog whistle.

I’ll say Pink Floyd fan (or antifa). Facist love to dog whistle, in my experience if he was, there will be more sign on him

As long as he isn't also sporting the Death in June merch