I think you should look at it in a different way.
Once someone plays Fertile Footsteps (Adventure part of Beanstalk Giant ), and you counter it, the spell does not resolve and should go to graveyard as it is generally a creature card (with an Adventure Sorcery part only).
That's my view on it. Maybe a judge could confirm.
November 27, 2019 8:45 a.m.
In the stated situation, Beanstalk Giant will be a creature card and unaffected by Dryad Militant . Unless cast as an adventure, all adventure cards are creature cards. If cast as an adventure, it will use its alternative characteristics.
715.4. In every zone except the stack, and while on the stack not as an Adventure, an adventurer card has only its normal characteristics.
November 27, 2019 6:33 p.m. Edited.
After rereading it, if you were countering Followed Footsteps, Dryad Militant would exile the countered Followed Footsteps.
715.3b While on the stack as an Adventure, the spell has only its alternative characteristics
On the stack, since it was cast as an adventure, Followed Footsteps only has its alternative characteristics. Therefore, it would be a sorcery card being put into the graveyard. Dryad Militant will see this and replace it going to the graveyard and put it into exile instead.