I don't see that there is a question here? You ran it correctly. Two pcs rolled better, but failed to see it. It's an ooze made of filth in a room that is all filth. Once it was sprung you ran the fight normally?
The pcs could have chosen to ready an action to Shoot the first thing that appears, up to them.
As the GM you can add things to the setting so it makes sense as to how the ooze was hiding, but I've already mentioned that it's a pile of filth in a room of filth. This also explains why the pcs wouldn't be able to hide in the same manner.