That looks like a Euphorbia. White sap should come out if and when the skin is broken. Be careful not to touch the sap or especially get it in your eyes. The plant needs more light. All those long, skinny, snakey stems are looking for more light. All the stems should be as thick as those lower ones, which were growing in strong light. The plant should "see" the sun for hours a day if it will be indoors.

Thank you for your feedback! It is an indoor plant, but is right on the windowsill of a south-facing window-I'm not quite sure how to get more light to it!

Maybe there's a tree blocking the light outside? Blinds or a curtain? The location you're describing should be good from the standpoint of light.