It's tempting to think that anything with the outer shape of a human would simply collapse in a heap if its skeleton were not there. That is surely true if the internal structure was the same as ours but with the bones simply non-existent.
However the human tongue, the elephant's trunk and the octopus's whole body work perfectly well without bones to support them. An octopus can squeeze through remarkably small spaces https://en.wikipedia.org/wiki/Octopus
I'd like my land-living approximately human-shaped aliens to be able to walk upright on land and disguise themselves as humans but also have the ability to get where humans can't by squeezing themselves through small apertures.
The question is - could a creature without a skeleton actually hold and balance itself upright on land for any appreciable length of time? What difficulties might it experience?
Video of octopus walking on land https://youtu.be/TFzpC_e44Tg?t=43
A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. https://en.wikipedia.org/wiki/Hydrostatic_skeleton
A muscular hydrostat is a biological structure found in animals. It is used to manipulate items (including food) or to move its host about and consists mainly of muscles with no skeletal support. https://en.wikipedia.org/wiki/Muscular_hydrostat