Per the description, there is no particular event that the Halo of Spores ability is actually a reaction to. Given that it's an Unearthed Arcana feature, it may be modified to match other reaction-based abilities and thus actually trigger as a reaction to something if/when it's properly published.
This forum post seems to list all the different reaction-based abilities/features. Many are tied to attacks or damage caused by the enemy (or by the character themselves), and some are triggered by nearby allies' abilities, but they do all seem to have some sort of trigger.
That said, even without a specified trigger, reactions work the same way: if/when you use a reaction (which can be on any turn, including yours), you can't use another one until the start of your next turn.
