Basically the exact same question as this one, but for 5e (since the other question is about Pathfinder).
A Horned Devil can inflict an "infernal wound" with its tail attack , which essentially causes bleed damage over time (bold only emphasis mine):
Tail. Melee Weapon Attack: +10 to hit, reach 10 ft., one target. Hit: 10 (1d8 + 6) piercing damage. If the target is a creature other than an undead or a construct, it must succeed on a DC 17 Constitution saving throw or lose 10 (3d6) hit points at the start of each of its turns due to an infernal wound. Each time the devil hits the wounded target with this attack, the damage dealt by the wound increases by 10 (3d6). Any creature can take an action to stanch the wound with a successful DC 12 Wisdom (Medicine) check. The wound also closes if the target receives magical healing.
A monk with the Way of the Open Hand monastic tradition has the Wholeness of Body feature:
Wholeness of Body
At 6th level, you gain the ability to heal yourself. As an action, you can regain hit points equal to three times your monk level. You must finish a long rest before you can use this feature again.
which doesn't explicitly say that it's magical healing, but the implication is that you can do this because of your Ki, which is described as magical under the "The Magic of Ki" section of the monk class (bold emphasis mine):
Monks make careful study of a magical energy that most monastic traditions call ki. This energy is an element of the magic that suffuses the multiverse—specifically, the element that flows through living bodies. Monks harness this power within themselves to create magical effects and exceed their bodies’ physical capabilities …
So, is a Way of the Open Hand monk's Wholeness of Body ability considered to be magical healing such that it can be used to close an infernal wound?
Best Answer
No, the healing is not magical
Only spells and things which are explicitly described as magic are considered magical. Monk effects mostly come from Ki, which is usually considered a non-dispellable form of innate magic. See this question about whether ki counts as magical for exceptions to the rule and a more in depth analysis of the Monk specifically.
This Sage Advice column from February 2016 and the corresponding section of the Sage Advice Compendium describe what counts as magical (See question "Is the breath weapon of a dragon magical?"):