Interestingly, there's nothing wrong in general with starting a syllable with ml-; it's fundamentally the same mouth motion as starting with bl- or pl-, both of which are common in English. But ml- isn't allowed.

This plays into a pet observation of mine, which is that an underappreciated constraint on the space of words that actually exist in a language -- as opposed to the space of words that could conceivably exist -- is that by and large they must descend from older words in an older form of the language, so that even if a word like "plailm" obeys the rules for modern English syllables, it can't exist because its precursor word would have violated the rules for older English sounds. (I don't know if this is actually true as applied to "plailm", but the phenomenon (of possible sounds failing to exist due to their precursors having been impossible) is real.)