The gemara quotes from braisa that if one fails to fence their vineyard thereby causing the adjacent produce owned by someone else to become assur as kelayim, the owner of the vineyard is responsible to pay for the damage. Tosafos questions, why is the owner of the vineyard liable to pay, it should qualify as a hezek sh'eino nikar - an unrecognizable damage, which is not considered a damage? Tosafos answers that even if the damage isn't recognizable in the object, so long as the context of the situation looks like a damage i.e. the vines growing near the produce without a fence separating, it is considered a hezek that is nikar and the owner is liable. Tosfaos asks, if this is considered "nikar", why do we considered it to be an unrecognizable damage when one takes a sheretz and places it on his friends taharos, there too the context of the situation should qualify as a hezek nikar? Tosafos answers that since tu'mah requires not only contact between the sheretz and the taharos, but also requires hechsher (fruits to become moist willingly to be susceptible to tu'mah), that aspect is still not recognizable and therefore qualifies as a hezek sh'eino nikar.
The question is, Tosafos just got finished saying that kelayim is not merely a situational issur of mixing produce and grapes. Kelayim is only created if the owner "wants it". Based on this, Tosafos explains that so long as the owner is doing whatever possible to build a fence, even though the kelayim grew .5% prior to the fence being erected, it is not considered a kelayim violation. Since kelayim also has it's own set of prerequisites to be assur - only if the owner fails to put in the effort of building the fence, which is not necessarily recognizable, we should consider kelayim a hezek sh'eino nikar, just as we consider tu'mah a hezek sh'eino nikar (due to the lack of recognition that it became huchshar l'kabel tu'mah)?
Tosafos apparently holds that by kelayim the issur is a metzius of growth. We don't require the consent of the owner to create the issur, just that if the owner makes an effort to build a fence and shows that he doesn't want the kelayim, the issur can be avoided. Tu'mah requires a positive act of hecsher to create the status of tu'mah, therefore it is considered eino nikar, but kelayim doesn't require a positive act to be assur (rather, a positive act to repair the fence will prevent the issur).