it either caused an XK Class Scenario and it somehow got contained and got sent to the current Foundation,
That's….not really how an XK-Class Scenario works. That is the designation for an end of the world event. There would be no Foundation left to contain it. Do you mean it got like, transferred between dimensions, or through time?
It seemingly appeared in a Foundation site contained
It appeared, already contained? That's awfully convenient, considering there wouldn't be much of a way for them to contain it otherwise.
Anything that comes into contact with it is absorbed into it and is added into its mass
Anything? Like, even air molecules? Because it seems like it would expand to the atmosphere and then the whole planet pretty dang fast if it absorbs everything. Does this process take a while? Are things killed when they're absorbed?
It seems like you have sort of a nebulous idea of what this thing both is and does, and you really need to have that solidly decided when bringing an idea in for critique. What you have currently is the "thing that does a thing" or "X that does Y" concept. An SCP article is much more than simply an anomalous item, being, place, or event. The true meat of the SCP is the story around the anomalous thing. The thing itself is a vehicle that the narrative is attached to. To give my best example from Series 1, look at SCP 093: The anomalous item itself is a rock that turns mirrors into interdimensional portals. The thing that makes this SCP shine is the story; the history of the dimension that it leads to.
We need to know more than just "it's a mass that absorbs everything." What story are you going to tell - How The Foundation found it, Who or what created it and why, did someone use it to some end? Is it sentient or sapient, or just a thing? (These are not all questions that must be answered, these are some possible narrative hooks.)
I also recommend reading a significant amount of the top-rated SCPs from the last several months, and focus on reading more recent SCPs rather than older series. Many early series SCPs would not last a day under current SCP standards, and a few are only still around because of their history with the wiki itself. (This does not apply to all early series SCPs, but many of these were written before the wiki's style had really been cemented.)
The anomalous object, while important, is not actually the main focus of the SCP. A good story, a compelling narrative, something that makes the reader feel something - that is the goal of an SCP article. What feeling do you want to evoke from the reader? Horror? Empathy? Sadness? You need to create a narrative around this object and its effects.
I am not saying this to discourage you, I just want you to understand the amount of effort that goes into creating a successful SCP.
If you can revise this idea with a story that you want to tell, we will be able to give you much more meaningful and constructive critique.
The anomalous item is the vehicle, the narrative is the person driving, the readers are the passengers. You can have the shiniest, most polished, most interesting vehicle in the world, but without the driver, the passengers aren't going anywhere. Where is the narrative taking the readers, and how is it getting them there?