Mescaline doesn't have "cactusy" taste. Especially when pure.
It is strongly bitter, not just slightly.
Mescaline HCl is usually in the form of powder rather than crystals.
Purely white mescaline is not very likely, as it would have to be either synthesized mescaline or very purified extracted mescaline. The synthesis is highly unlikely and most people don't fully purify extracted mescaline HCl (although it is possible), so some coloration (like shade of white) would be more likely.
So the odds you have mescaline are very low, I would say less than 1:20, if you don't know its real source.
Imho the best thing you can do, if you don't have any reagents, is to dispose of that.
Acceptance of the fact that our reality is not real doesn't in fact mean it is not real. It just leads to better understanding what real means.