Hyphens often disappear over time as compound adjectives become more established phrases, so it's no surprise that the hyphen in easy going is disappearing in some places.
The Ngram for this is pretty nifty; as the two-word phrase rose in popularity, authors began to dispense with the hyphen, either by replacing it with a space, or using a single-word term:
Fact is, you can find both versions in published works, so there's no definitive answer. Neither would be considered incorrect. In fact, some dictionaries (like M-W) show it as a compound word (easygoing), while other dictionaries (like Collins) retain the hyphen (easy-going). Also, one dictionary (WordWeb) lists one form as a noun (with the space) and the other as an adjective (as one word).