Gelli Baff is a powder that you pour into your kid's tub to turn the water into colored slime.
The goo is a completely harmless powder that soaks up 400 times it's own weight in water. When the fun's all done, add the disolver sachet and the goo dissapears.Link (Via Play Library) Alex says:
Gelli Baff is just SPA (sodium polyacrylate). Sounds like they've found a fun way of marketing sodium polyacrylate, a non-toxic, water-absorbing polymer that's used in everything from disposable diapers to agriculture. You can buy a whole pound of it from Steve Spangler's site for $20, as well as SPA in granular form, which can be used as fake snow.
A lot of other substances work well for "busting" the slime once you're done playing, table salt is one, but lots of other household products will do it as well, try some out!