If someone is just a sex offender and walking around and not trying to make contact with children or trying to harm anyone then calling the police won't do much.
It's not illegal for the sex offender to walk around or be in an area.
But if you think he's about to do something and he looks suspicious and is acting suspicious around people or children then it would probably be a good idea to have the police check them out.
You could actually be saving a kids life or protecting them from being harmed.
The police can come and make sure he's not doing anything wrong and make sure the man is not doing something illegal.
But he can't be arrested simply for being a registered sex offender.