facebook bug?

I was on someones Facebook about page on safari when I noticed that at the bottom of the page near the toolbar there would be a white text that read “ISUD” it would follow the bottom of my screen whenever I scrolled and would leave when I refresh but would come back again if i refreshed a couple more times, has this happened to anyone else and is it a bug/cause for concern? It started happening after I pressed a gofundme link but it would happen randomly on other tabs too (on the same Facebook page) so I’m not sure if that’s related.