If Alex is his first name, then no. You put "Mr/Mrs/Miss/Ms" before the person's surname. If his name is Alex Smith, you can start the letter either "Dear Mr Smith" or "Dear Alex". If it's a business letter, you would only start it with "Dear Alex" if you already know the person very well, you regularly address each other using your first names and (in my opinion) only if it's a letter that's probably only going to be read by Alex.
Don't use "Good day!" There is no need for any salutation after "Dear Alex". Also, an exclamation mark would be inappropriate in a business letter.
I suggest starting with something like:
Dear Alex (or "Dear Mr Smith")
I am writing in response to your letter of ...
Thank you very much for the tour of your facilities last week ...
or whatever is appropriate to your context.
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