This base type corresponds to an adjectival derivation (cf. Georges under pellīceus) of the Latin noun pĕllis 'skin, fur, pelt'. The metaphorical meaning 'cream; layer of cream on boiled milk' is of similar origin as *nīta 'cloth' or pannus 'tissue', which have developed quite similarly semantically. Moreover, extended forms of the Latin root pĕllis have occasionally also taken on the meaning of 'cream' (cf. pĕllis) within VA's research area.
Note that the Alemannic forms are consistently masculine, while the Romansh pleʧɑ 'cream' (in Val Müstair in Grisons) is feminine and thus similar to the fra. pelisse and ita. pelliccia 'fur' (cf. FEW, 8, 162–164, under pĕllīceus). The Alemannic forms therefore seem to be secondary developments of a loanword of the German type Pelz that had already been adapted in gender. (It, of course, ultimately goes back to Latin pĕllīceus; cf. Kluge, 692 and AWB, under pelliz.) These forms are thus not relics from their local Romanic substrate, which, in terms of gender, would have to correspond more to the feminine pleʧɑ mentioned above.