The end of our bath isn't sealed properly, and without doubt splashes
have made their way to the concrete below over the years.

But... is this potentially something nasty, or just salts /
efflorescence as a result ?

http://www.coreutilities.co.uk/damp.jpg

There's no sign of it elsewhere around the bath, just immediately
under the place it'd catch the odd bow-wave :-}

Thanks :-}