Could be anything, I suppose, but presumably not meant to be there. Nothing much to do but monitor the situation, keeping a packed overnight bag by the bed if you're really nervous. But the fact that it's adhered to the glass rather than mixing with the drink suggests you're probably okay.
I'd expect any residue from something that mingled with the fluid to be on the bottom of the bottle rather than the side, like the muck you sometimes still get with red wine. The fact that it's still there suggests your mixer didn't mix with it. By all means invite trading standards to analyse it, though.