Line-by-line:
SCP-XXXX is held within a 5x5 meter lead shielded containment chamber located at site ██.
I'd use "is to be" instead of just "is".
5x5 is a two-dimensional space. You need a third measurement for 3D environments.
Why 5x5? Would containment fail if it were 6x6 or 4x4? Generally speaking, you should only provide a measurement if it's absolutely necessary for containment.
Proper formatting is "Site-XX".
I'd just give a Site number. Here is a list of secure locations. For this scip, I'd recommend Site-17, but there are a few that could work.
SCP-XXXX is an anomalous vaguely human sized entity of unknown shape and colour which appears to not require nourishment or water intake to survive.
Props on your spelling of "colour". Despite being an American, I hate American spelling.
"human sized" should be hyphenated: "human-sized".
You don't need to say a scip is anomalous; they all are.
Run-on, try separating: "SCP-XXXX is a vaguely human-sized entity of unknown shape and colour. Observation has thus far determined the entity to need no nutritional sustenance to survive."
The entity is not hostile however is capable of movement, moving between different corners of its containment chamber in irregular intervals of time with no apparent reason for doing so.
Hostility and movement are unrelated, so you should separate into two statements: "SCP-XXXX regularly moves between the corners of its containment chamber in irregular intervals; the reason for this is unknown. It has demonstrated no hostility, though relative lack of documented human contact may be the limiting factor."
SCP-XXXX has the anomalous property of being completely physically indescribable as any and all individuals asked to describe the entity after viewing it either in writing or verbally find themselves unable to form an accurate description that makes sense to the listener in concordance with our reality.
This is a substantial run-on. Try splitting it up: "SCP-XXXX's anomalous properties render it completely indescribable. Any and all viewers tasked with describing the entity, whether verbally or in writing, find themselves unable to form an accurate description in accordance with our reality."
It isn’t that SCP-XXXX lacks a physical appearance or that individuals simply forget what the entity looked like after viewing it as individuals who have viewed SCP-XXXX either through a photograph or in person do report seeing something they are just unable to describe exactly what it was that they saw.
Run-on with non-scientific tone. Try: "Brain scans and survey of viewers indicate no memory-affecting mechanism, rather, it seems participants simply lack the proper descriptive abilities to sufficiently describe the entity."
Conceptually, the main issue is the lack of narrative. It's an unwritten rule that Series IV scips need some form of self-contained story, and there's none of that here. If I were you, I'd expand on the discovery and initial containment - think of a typical detective manhunt-type story, with the added twist that the target of the investigation can't be described. Alternatively, I could also see a decent story about researchers driving themselves to near-insanity trying to get a good description of the thing.
As a related issue, the reader's sense of intrigue is negatively affected by the lack of development. The concept is seemingly tailor-made for mystery, but there's no example of an attempted description or anything of the sort to make the reader ask questions. As you develop the narrative, be sure to hold on to that sense of mystery.
Overall, I think the
Ideas and Brainstorming forum will be able to help you further develop the concept and give you a good start narrative-wise.
Hope I could help, best of luck!
