a code is visible in the header

It looks like you have added some CSS to the header code injection without telling the browser that it is styling and it is rendering it as text. 

You need to find where that text is, it will either be in Website Settings -> Advanced -> Code Injection OR under Advanced on that page settings.

Then either remove it from there and place in Custom CSS OR wrap the text in <style></style> tags so browsers know that it is styling and not render it as plain text.