can you fish them out? it looks like pieces of a gasket or something like that. Stop-leak usually works by plugging a hole (so as the coolant is pushed out of the hole, it builds up and forms a “patch” of goo over the hole to seal it. It won’t generally form large chunks in your system.
If they are all just loose pieces in the radiator, I would be tempted to remove it from the car, flip it over and try and flush it out (maybe block the upper radiator hose attachment and fill from the bottom, then pull the upper radiator hose and/or cap and see if the pieces come out as the water rushes out) – it doesn’t look like too much of an obstruction, but getting it out of there is probably better than leaving it in