General tips from an active Mormon here:
• As others have said, if your goal is to convert them to Catholicism, it’s not going to happen. Not that Mormons can’t be converted by they are at a time of their lives where they literally focused on converting people to Mormonism so any Catholic faith that starts to emerge will be weeded out while on their mission. Now, you can plant seeds to blossom post-mission, but you won’t see it now.
• A campaign to promote interfaith understanding and appreciation will go further than a pro-Catholicism campaign (speaking time and attention-wise), and also be better for Catholic appreciation and planting post-mission seeds and
• Remember they are ~19, and have all the pros and cons that come with it. Speaking generally- pros: generally a child-like devotion to Christ, cons: they are young and don’t know everything and can be immature. Then again, grandparents don’t know everything and can also be immature.
• Every ~3 months, Mormon missionaries are transferred to new areas (this is to help keep things fresh). So, if one moves while you’re in the course of talking, don’t think it’s because of you.
•** Speaking culturally: Catholics tend to be very intellectual about their faith, relying on logic arguments and tomes of old books. Mormons tend to focus on a faith that is lived and how the Spirit is felt in their lives—trying to walk as disciples of Christ. Not that both “schools of thought” don’t exist in both faiths, but I’ve noticed a marked difference in emphasis**.
If you have any questions or want to chat, feel free to PM me. I’m an open book about everything and have zero proselytizing intentions (I’m a huge promoter of inter-faith appreciation).