Identity on freelancing site will affect to my current working life?
I had a full-time marketing job working in my companies corporate division and the entire time I worked for them I also freelanced. DISCLAIMER: Just becuase I had a positive experience does not mean you will too.
My freelance work helped me in the corporate setting as I didn't have as much freedom to try new things for a multi-million dollar company compared to smaller companies that I worked with. I was able to use that knowledge and experience to improve my performance in the office and they were happy about it.
At a different position I had (different company as well) when they found out I was freelancing they offered me a raise (this isn't always going to happen). They believed I was freelancing becuase I couldn't afford to live off of what they were paying me.
I don't know where you are located but I work in the US and the marketing team and dev team I work with mostly have contracts with other companies as well as being full-time for this company. This has helped the company grow substantially in the past few years.
Really, it all depends on how your company views this. I personally can't imagine any company thinking poorly of an employee that also freelanced on the side unless you're working for competitors or using company assets (such as code) to fulfill those contracts.
Should I use fake identity? And is it Valid? (I think it is bad one.)
You can work under a fake name as @PeterMV mentioned but you'll need to create a life around that person. Even using sites like Upwork & freelancer my LinkedIn profile gets checked before I get hired for most of my high-paying contracts. Companies want to hire someone who is real and is going to deliver quality work. Having a name with no background to it is kinda sketchy.
Some sites will have you verify your identity but as to the process that all sites have to do such a thing, I'm not 100% sure. Some sites do let you register as a company instead as an individual.
Is there any other option for me to work safe and good?
You may want to just work under your real name and if your company does have an issue with it (if they find out) then you should explain why you're freelancing. As a developer there's so much out there for you to learn and that alone is a great reason. Maybe your company only uses Swift but you want to explore possible advantages of using Objective-C, or building iOS apps without Xcode, etc.
At the end of the day, it's up to you whether you want to use a real or fake name and each has it's disadvantages. There are good reasons as to why you'd want to use a fake name I'm also not sure you own anything you write if the developer is a fake name and there's no solid proof that you're that person (I would look into your local and federal laws).