subtle

(s) difficult to detect or grasp by the mind or analyze
- his whole attitude had undergone a subtle change
- a subtle difference
- that elusive thing the soul
elusive,
(s) able to make fine distinctions
- a subtle mind
(s) working or spreading in a hidden and usually injurious way
- glaucoma is an insidious disease
- a subtle poison
pernicious, insidious,

elusive pernicious insidious

subtly

subtlety