Hey Rocco,
Welcome to the forum and congrats with your WEPS!
I don’t know how many paste there is still left on your strop, but you can scrape off excess paste with a knife (just don’t cut into the strop). I’ve never tried to further clean my strops, but water, rubbing alcohol or an eraser might be your best options. (The eraser in the same way it can sometimes be used to remove glue.)
But… I don’t think you have to be overly wary about cross-contamination. You accidentally put 0.5 micron paste on the 1 micron strop. If you clean this strop reasonably well and then apply the 1 micron paste, there might still be some 0.5 micron particles left behind. But these won’t hinder the 1 micron particles. The 1 micron paste should work normally.
It would be different if it were the other way around: 1 micron particles would definitely contaminate the 0.5 micron paste.