What is the term for a beginner who is already very good?

Solution 1:

You can use the word "natural" either as an adjective or as a noun, to describe the person and their skill or the person themself.

natural (adjective)
attributive (of a person) having an innate skill or quality.
‘he was a natural entertainer’

natural (noun) A person having an innate talent for a particular task or activity.
‘she was a natural for television work’
Oxford Living Dictionaries

Another is the word "gifted".

Having exceptional talent or natural ability.
‘a gifted amateur musician’
Oxford Living Dictionaries

If you want another noun describing the person, then "prodigy"

prodigy
often with modifier A young person with exceptional qualities or abilities.
‘a Russian pianist who was a child prodigy in his day’
Oxford Living Dictionary

More often than not prodigy is used with minors or children.

Solution 2:

quick learner
"He was a pragmatist and quick learner. He was skilled at picking up ideas from others and making them his own." (Lawrence M. O’Rourke, Row House to White House, 2012, p.3319)

"The quick learner has what is called in ordinary terminology, the power of concentration. All the available cerebral energy seems to participate in ..." (William Henry Pyle - 1921)

"You had no experience as a woman, but you were full of spirit and a quick learner." (Gary Jonas - 2011)

Also, fast learner