Those are two different forms of two different prepositions…
- air means on him, it is the 3rd singular masculine form of air on, tha falt snog air literally means nice hair is on him – you use air on here, because nice hair is some attribute of his, not part of his possessions (he didn’t buy nice hair, doesn’t keep it in his closet, but rather they are a part of who he is),
- aici means at her, it is the 3rd singular feminine form of aig at, tha cat aice would mean she has a cat, tha càr agam would mean I have a car, tha taigh aige would mean he has a house, etc.
See this comment of mine under the "Do I have a small head?" sentence for the difference between air and aig in the sentences involving the verb to have in English and also Aig, air agus ann an or The severed head on the Akerbeltz wiki.
As for the conjugated forms of prepositions (air, oirre, orm, ort, etc. and aige, aice, agam, agad, etc.) you’ll need to read the tips and notes to the lessons – they are described in the lessons where they are first introduced. You can access the notes in the web browser version of Duolingo (unfortunately not in the mobile app), or on the duome.eu website.