Hello Frogsrquete,
Ignore the message. Web browsers don't have the ability to scan your iPhone for viruses. iPhones, by there lockdown nature, are extremely hard to hack. Unless you are a target of a government and/or have jailbroken your iPhone, you should be safe. The only danger is installing the app. When you do, they will falsely claim there are "viruses" on your iPhone and request that you charge a sum of money for the removal. Believe me, I've almost fallen for it. Also, robux to USD converters aren't legit. Robux are just in-game currency and they themselves don't have monetary value.
Cheers,
Jonasilver137