The problem with this card is that Aflatoxin doesn't make for a great reusable ability. Most other Nymphs don't have this problem - you're as happy about the second Antimatter as you are about the first, and same with Precog, Rage Pot, Basilisk Blood, etc. So with those you are more willing to risk getting no effect at all (because your Nymph might just get CC'ed).
However, let's look at Aflatoxin. This card has two uses - firstly to kill a creature, but it's really inefficient at that, there are much better options for it. Secondly to block the opponents field with cells, which will usually be the reason for playing Aflatoxin. And that's exactly the reason why Grey Nymph isn't used - the field cannot get fuller than full. Most likely the further activations will only be useful for slowly clearing the field of the remaining normal creatures.
I think to be viable, the cost for playing Nymph + using ability for the first time needs to be as expensive or even cheaper than the original card (which unfortunately is a bit overcosted too). That or it needs to be given some extra value by getting significant ATK/HP stats.