What kind of lava tube feature is Cleopatra's Grave?
It's just average ropy lava called pahoehoe by geologists. It's similar to dough so it easily forms ropes like the ones around the side and bulbs like the central area which is similar to a pillow except that it's called a toe. 

The colder black lava flows like ropes and the hotter red lava is smooth. 


This is also from Cleopatra probably: https://media.gettyimages.com/photos/ropey-lava-from-volcanic-eruption-picture-id200432059-001

Another good image of a toe:

Source: Oregon State University