What is this in the beef stew?
This is protein that is coagulating in the water simmer environment.

Most beef stew and braises in general start with a high heat sear of the meat, often dusted with flour or starch to begin a thickener.

Simmering any animal protein in water will produce this curd like gunk. If you continue, and add other ingredients, and reduce the liquid, it will eventually incorporate,more or less, bt will look like that for a while.

Wondering what recipe or, more importantly, technique you are trying to use.

If you provide more details about timeline and technique, more can be said, but simmering meat in water makes this curdy mess.