Authenticity of memory alert

Is there one or more viruses than can cause your iPhone to create a very Apple branding looking message that says your device is about to run out of memory and starts a 2 minute countdown to download a cleaner app from App Store? It appears real but my phone memory is not even close to being full. If not a virus, how did this “promotion” get in my phone? How do I remove it? And how do I prevent this and similar “viruses??) from getting in my apple devices?