If the surface here is "mostly mud" then I'd probably have gone with "surface=mud" (and "sand" for "mostly sand" nearby).
If mutiple values are needed, then senicolons are usually used as separators.
Data consumers have the choice whether to use the whole value or just the part up to the semicolon.
You can see which tag values are used by which projects by looking at the "projects" tag at taginfo.osm.org.
The maps I produce show tidal and non-tidal mud and sand as different patterns, and because "tidalflat" is a bit vague, use the surface value.
Generally speaking, if a rarely-used value can be obviously replaced with another value that means exactly the same thing it makes sense to do so, but it doesn't really make sense to remove a rare value altogether.
Presumably you've had a look at different sets of imagery here and already have an idea that "yes, that's more like mud than sand" (or vice versa) so should be in a position to decide which - or if it is not clear, "surface=mud;sand".
