If the slime returns, the i suggest you boil the decorations. Put them into a pot filled with water and let it boil five or ten minutes. This kills anything that might be living on the thing, and should prevent it from happening anymore. I've had a similar problem with some rocks that I put in after only having rinsed them. The film seemed to be having a negative effect on my fish - some torn fins suddenly. After I boiled the rocks, though, i never had the same problem again.
You can also try disinfecting them with a bleach solution - soaking them in a solution of one part bleach to ten parts water for a few minutes. And rinse off really well afterwards. But I'm not sure about this method though, I'd recommend boiling them, it's worked very well for me.
