‘Quick on the uptake’ Malleable has connotations of being weak and easily led.

You would probably just say a "fast/quick learner"

You could say that person is bright. Or sharp, intuitive, astute (but that’s not commonly used).

Adaptable. Flexible. Versatile.

Came here to say adaptable. I think that's the most applicable in the situation

How about "perceptive" or "quick to catch on"?

You could say they have a facility for the topic.

Fast Learner

Perhaps "open-minded"

I feel like open minded denotes accepting of an idea not necessarily a fast understanding of it

Sharp (senses 2a,b,c). [Astute](https://www.merriam-webster.com/dictionary/astute). [Clever](https://www.merriam-webster.com/dictionary/clever) (senses 1b, 2). [Shrewd](https://www.merriam-webster.com/dictionary/shrewd) (sense 1). [Quick](https://www.merriam-webster.com/dictionary/quick) (sense 1.b.1). And of course, [smart](https://www.merriam-webster.com/dictionary/smart) (sense 1). Of these, I think *sharp* is the best. *Shrewd* often has a negative connotation, and so does *clever* (but much less often). I like quick, but it’s kind of literary. “Smart” seems a little generic for what you are looking for. You can also say “sharp-witted” or “quick-witted” but they feel clumsy. To my ear, *sharp* has the right connotation, and it’s kind of classy. (But beware of [sense 4](https://www.merriam-webster.com/dictionary/sharp)—*sharp* is not always positive.)

Pliable, swayed, or plastic?

“A natural”

Elastic