#1 - June 25, 2016, 10:10 p.m.
It appears that it is not castable while moving when at or below 25 maelstrom, but becomes castable while moving when at or above 26 maelstrom.
Edit: The above numbers are not final. Below is a screenshot where Healing Surge was not castable while moving despite having 27 maelstrom. "Can't do that while moving" error text can be seen.