I'd like to know whether I should use the word "geek" as an adjective or noun?
The word "geek" is a noun, just like "dog" or "cat" is. It can be used as an adjective though, but in a slightly altered way:
He looked geeky with those glasses on, holding that laptop firmly under his arm.
Here, geeky is being used as an adjective, meaning that person looks like a geek because of the arguments given afterwards.
Any noun can be used as an adjective without changing its form:
Geekspeak, geek-speak, and geek speak are three compounds that consist of two nouns each. The first noun modifies the second by telling you what kind of "speak" it is. There are many different types, e.g., business speak and corporate speak etc.
I can't see why it'd be wrong to use "Majid Geek" as a handle.
In "Majid Geek", you say "Majid is a name". In this phrase, "Majid" functions as an adjective because it tells us what kind of geek you are. But because "Majid" is an Arabic name, most native Anglophones won't understand what it is or what it means. I don't and didn't know that it was an Arabic name.
Also, the word order isn't English. If you want "geek" to be an adjective, English word order is "Geek Majid". But even if the name were "Geek Thatcher", "Geek would probably be considered only a noun that functions as a name (proper noun), not an adjective in that two-word phrase. To be an adjective in a name, it'd have to be Geeky Majid, but even then it would be an adjective that functions as a proper noun (name).
In geekmobile and geekthink, geek functions as an adjective. It's all a matter of context.