User blog comment:FairyRaven/Fairy Tail Chapter 427: Pitched Underground Battle, Review and Commentary/@comment-31.132.65.54-20150408015850/@comment-7418318-20150411071719

The reason why Iced Shell couldn't be affected with Arc of Time was because the Ice of Iced Shell wasn't Ice, but a transformed living being. That is not the case with Silver's Magic, thus it's actually quite likely that Arc of Time would affect it.

In addition during the timeskip Ultear learned to have at least some kind of effect on living beings as was shown by her unlocking Second Origin in living people and the use of Last Ages. So it's even theoretically possible that a sufficiently skilled user of Arc of Time could have undone Iced Shell, but Ultear just wasn't able to do it back in X784. Unlikely, but possible I would think.