Flail Snail and Invisible Stalker are valid options for conjure elemental
Choose an area of air, earth, fire or water that fills a 10-foot cube within reach. An elementary of Challenge rating 5 or lower appropriate for the chosen area appears …
First, both creatures are below CR 5 (the Flail Snail is CR 3 and the Invisible Stalker is CR 4), so they are valid options for this spell.
Second, both creatures are associated with an element. The snails are associated with the earth (Volo's guide to monsterspg. 144):
A snail flail is a creature of elemental earth …
And the invisible Stalkers are associated with the air (Monster manualpg. 192):
An invisible stalker is an elemental of air.