Plum

Spirit Beast comes in many shapes, sizes, unique powers, abilities, and strange physiques. Whenever a human binds his soul with any one of those Spirit Beast then there's a huge chance of being gifted with an ability related to that particular spirit Beast in return. When Alex bonded with Plum, he benefited by getting a part of the Slime's peculiar physique as the gift.
Alex could now in exchange for some Qi, grow out, elongate and shape the muscles into limbs or other eccentric shapes and control them just like any other part of his body, similar to how Plum can extend out tentacles from within its Semi-liquid form.
Alex can bend his muscles into any shape he desires but to a certain limit and having control over it just like any other limb on his body.
If Alex wants to, then he can grow out two extra arms or legs similar to the real ones although boneless, which makes them much more flexible then the existing ones. He can, of course, control the rigidness of the two extra limbs to be the same as the actual ones if he so desires and can even harden them much more to a certain degree.
There are some restrictions though. He cannot stretch his muscles and the skin around it out indefinitely. There's a certain range to which his muscles can stretch up to which is approximately four to five meters.
Nor can he grow innumerable tentacles or even more than two as big as the size of his arms out at the same time from his body like how Plum does it. Alex can however only do so if he keeps their sizes as small as his fingers after which he can use and control them like vines.