Interesting concept.
But, no. Cards which offer some sort of global cleansing of the game table are going to be shot down by 9 out of 10 people on this forum. So let me be the 1/10 to offer you some advice to change the idea before the other 9/10 get here eh?
Firstly, I'd change the name... Alfatoxin was originally going to be called Cancer, but it was thought that could be a subject some people wouldn't like coming up in a game - so it was renamed Aflatoxin, and the Malignant Cells changed from Cancerous Cells. So, might just be me nitpicking, but a change from Chemotherapy and Radiotherapy would be nice.
Secondly, instead of a simple 'Kill all creatures', make it do 2 damage to all enemy creatures - then make it cost 7 Quanta (5 Quanta upgraded). The logic behind this is, Rain of Fire does 3 damage to all enemy creatures and costs 7 Fire Quanta, this does slightly less damage to all enemy creatures but if those creatures are Malignant Cells, it gives some decent healing properties.
Thirdly, not so sure if this sort of mechanic (if creature dies and is malignant cell then add hp / if creature dies and is not malignant cell lose hp equal to hp of now-dead creature) can be implemented easily, this is a very case-specific card. You would need to run aflatoxins and this to make it work.
So, good luck
, open the flame-gates!