An entry evidently generated by Thucydides 4.100.1 and/or 8.3.1, the only two (other) attested instances of this genitive. It refers there to the Malian Gulf in north-central Greece; see Barrington Atlas map 55 grid D3. The form was noteworthy because elsewhere Thuc. uses others, e.g. Meliakos in 3.96.3.
Given Greek dialects, there was -- and remains -- great scope for confusion between Malis and the Aegean island of Malos/Melos (together at mu 927) and their respective cognates. See generally LSJ s.v. *mhlieu/s and *mh/lios. The latter is the more frequently mentioned in the Suda: see e.g. delta 523, delta 524, mu 455, mu 923, sigma 830, upsilon 217, phi 381.