If it's soft, it is rotted and would be best removed from your planting. If any of the rhizomes in the clump are still firm and showing signs of growth, leave them and remove the rest of the rotten clump.

Thank you for your reply. Is there a reason why this one is rotted and the adjacent ones were not?

Some irises are just less disease-resistant and more susceptible to rot infections. It's also possible that the infected clump suffered some sort of physical injury that left it vulnerable to bacterial invasion.

Thanks again! I wanted to make sure I don't repeat my mistake again. I did not plant these rhizomes very deep for the fear of rot. About 1/2 of the rhizomes are visible. In early spring, I notice that the one was all out of soil - maybe from the froze/thaw cycle. I added some soil to it. Maybe I should of mulch them during first winter.