Guernsey made a poor start as Adam Martel was run out for one at the end of the first over.
Martin-Dale Bradley and Ben Fitchet put on 33 for the second wicket before the former was caught by former Australia Test player Joe Burns for 18.
Fitchet was trapped lbw by Harry Manenti two balls later as Guernsey slumped to 36-3 off 5.4 overs.
Matt Stokes went on to make 13 while Tom Nightingale’s 35 provided the backbone of Guernsey’s innings.
But after Nightingale was caught and bowled by Benjamin Manenti for 35, Guernsey lost three wickets for just one run to leave them on 101-8 off 16.4 overs.
Ollie Nightingale scored 17 before he was caught off the penultimate ball of the innings as the Sarnians ended on 122-9.
Luke Bichard took Emilio Gay’s wicket in the first over for eight as Italy’s reply began with a stutter.
But they recovered well as Mosca and Burns (30) put on 62 for the second wicket before Harry Manenti hit 22 off 22 balls.
Manenti was dismissed after 13.3 overs but Mosca guided the Italians home with an impressive 37-ball knock that included three sixes and six fours.