The only way to prevent network monitoring is to connect to a VPN or an unmonitored network, so if you wanna get rid of that without connecting to a VPN you'll have to connect to a different network entirely, if it still displays that message when connected to an LTE network then your carrier of choice is monitoring your network usage which is kinda sketchy but phone companies can get away with all kinds of crazy stuff.
Either way I'd stay connected to a VPN if that message continues to be displayed unless you are connected to a VPN otherwise some not so favourable people could get ahold of your personal info like passwords and credit cards and stuff.
It is a network monitoring warning added by Google as part of the Android KitKat (4.4) security enhancements. This warning indicates that a device has at least one user-installed certificate, which could be used by malware to monitor encrypted network traffic.
