I am new here as well and want to offer some input to flesh this one out, if I may.
Special Containment Procedures: The organism must be stored in a cubic, glass container.
I think adding in some dimensions for the container would be good.
To the unaided eye, the organism appears to be a deformed hand that is oozing a liquid from the fingertips. Later inspection revealed the liquid-like substance to be groups of tendrils of an organic material. The tendrils emerging from the hand measure only 4 picometers thick and can reach up to 2 cm in length. The liquid appearance of the organic materials coming out of SCP-XXXX is attributed to the size and quantity of the tendrils that move together.
I made some corrections to the above and rearranged some things to tighten it up a bit.
When within range of its tendrils, a chunk of an object will be removed by the ends of the tendrils and pulled toward the palm of SCP-XXXX. Once in the palm, the subject will use its tendrils to mold the chunk into the shape of a body part. The properties of the tendrils are still under investigation, but they appear to be able to change the any solid matter to a malleable form. Footage from a thermal camera taken during initial screening and study showed the ends of the tendrils are very high in temperature. However, no experiments proposed from this data have been successful in determining the process of SCP-XXXX's ability to break down matter.
What does it do with the finished parts? Are they normal human parts, or something else? Are they functional? Do they have any unusual characteristics or exhibit any sort of strange phenomenon? What materials have been tried (besides glass) and what were the effects? Does it toss them away or try to integrate them into itself somehow? I know you address the last one in the "observations", but explaining that in the "description" section would be a good idea.
Origin:The wrist of the hand shows markings that suggest SCP-XXXX was removed from another organism at some time prior to its discovery in 1986. Current data concludes that SCP-XXXX consciously separated from the original mass. No traces of the original organism were found near SCP-XXXX at ██ █████.
I think you should flesh this part out just a bit more, but that's just my .02. It has a lot of potential for the creepy factor.
-Although the subject can break down any matter, it appears to struggle with glass. While the nature of its tendrils is still being researched, it is assumed that the organism will have difficulties interacting with all transparent materials. An experiment will be conducted in March of 2013 to investigate the subject's interactions with different materials.
-Limbs constructed by the subject begin to break down when existing materials provided are used up. An experiment that will provide the organism with enough materials to complete the body will be conducted in January of 2013.
-SCP-XXXX appears to always construct the same limbs in the same order from any materials provided, organic or not. Further studies are being planned.
What happened when it tried it with glass? Also, if it was me, I would black out the months/years.
I would definitely recommend perhaps reading up on anatomy and anatomical mutations for some brainfood. There is plenty of gross/horrifying/creepy stuff in regards to that in the real world to draw inspiration from.
I hope this was somehow helpful to you. Good luck!