A good opportunity to calculate through the numbers!
- Let's use the number of 1 g calcium per litre of milk, that's 25 mmol/L. But let's also keep in mind that much of the calcium in milk isn't free-floating calcium ions, but bound to phosphate.
- My local tap water (hard, but not very hard) has some 2.7 mmol/L hardness content (Ca + Mg combined)
- Making a 38% lye solution from one litre of liquid (typically: distilled water), like ussual for soapmaking, holds 15000 mmol of NaOH, liberating the same 15000 mmol of fatty acid anions during saponification.
Yes, calcium ions (from milk, tap water or whatever), will form soap scum. But the mere order of magnitude of the numbers already points to where this is going: With milk, only one in 300 soap molecules will have the chance to get in contact with a Ca²⁺ (worst-case scenario if the calcium phosphate is treated like fully dissolved, which I doubt). That's a really tiny amount, that won't affect soap performance at all. Many people put more titanium dioxide into the soap deliberately!
Additionally, the concentration of free calcium can (and should) be lowered with chelators like citrate, gluconate, or EDTA.
With (my) tap water, the scum precipitation is lower by another factor of 10.
Note, however, that tap water does not only contain “harmless but annoying” trace metals (Ca, Mg), but more often than not also traces of much more troublesome metals (Fe, Cu, Zn, maybe even Pb) from the plumbing.
Any chance to get these into the soap should be avoided in the first place!
tl;dr: Calcium in milk (or tap water) does produce soap scum, but in so minute amounts that it is totally overshadowed by all the other benefical properties of milk additions.
In any case, the overwhelming amount of soap scum forms during
rinse, i. e. when the dissolved soap comes into contact with plenty of tap water
after it has done its job.
