He won't. He's in a war with Ackman. If he sells HLF at the current price, he will make a decent profit. However, it means he has admitted his loss to Ackman. Icahn is so rich that he cares about power more than money now.
Also, by buying FNMA, he will be on the same boat with Ackman and that's not going to happen, unless he prefers money over self-esteem.