I knew that you are becoming someone.
In this sentence we are talking about two different time-zones:
In the past:
I knew something.
The thing I knew in those past days is an event in the relative future. Relative future means, that is was a future event from the past point of view.
You are becoming someone now.
This is happening now. This present is the future of the past.
That you are becoming someone, is a fact now. So there is no reason to use a conjunctive form. Indicative is the right choice, at least when you want to express in German, that you are talking about a present fact.
But is is allowed to use conjunctive. If you use conjunctive, you express, that it was a future possibility in the past.
Correction (Thank you, Numeri, for your comment)
Sorry, I missed the word "mal" in the original sentence. Everything I said above is true, when you omit the word "mal".
German present tense (Präsens) is different to English present tense. You can use Präsens to express ...
- something that happens right now
Du liest diesen Satz.
- something that happens often, at any time, even if it maybe doesn't happen right now
Jürgen betrinkt sich an jedem seiner Geburtstage.
- something that is fact, independent from time
Kugeln sind rund.
So far, so clear, but German Präsens also can do something you wouldn't expect from a present tense:
- an historic event that happened in the past
Als die Berliner Mauer fällt, sitzt Frau Merkel in der Sauna.
- something that will happen in the future
Ich gehe morgen zum Arzt.
Because of this flexibility of Präsens it is often unclear, at which time something is happening when you use this tense. For example:
Aus dir wird etwas.
This can mean:
present: You are becoming someone right now.
future: You will become someone sometime in the future.
But German has some temporal adverbs, that you can use to make it clearer, and mal is one of them. Mal is a colloquial short version of einmal. This means once, but line English once it does not always mean one times (in contrast to two times, three times, ...). It also can means sometimes.
When you use the adverb einmal or mal together with Präsens it refers always to something that will happen in the future.
Aus dir wird mal etwas.
wrong (present): You are becoming someone right now.
correct (future): You will become someone sometime in the future.