Sounds like it "broke", as can happen with any dairy product. The protein in the kefir coagulated into the strings you are seeing. If you decide to try it again, try the highest fat kefir you can find, minimize acid in the dish, and incorporate the kefir at a relatively low temperature.
The dynamics of the Grain growth are not currently well understood, much less its formation. Some researchers attempted to create grains in laboratory, but failed.
There are over thirty or fourty strains of microbes in the kefir, and their ecological relationships are very complex. One could call the whole kefir culture an ecosystem of its own.
The grains grow either because the matrixes of existing grains entice the microbes to create more grain matrix, or because there are specialized microorganisms inside the grain that are responsible for creating more grain.
What is known is that the grains need Kefiran to grow, and Kefiran needs Alcohol in its synthesis. Commercial cultures of kefir often omit the alcohol-creating yeast for obvious reasons, so it too adds to the impossibility of getting Kefir grains from commercial kefir.
Either way, the fact is there grains do not come into being on their own. New grains are split from existing grains.
There is a small possibility that a kefir culture from grains that was strained could form new grains, but its not confirmed.
The commercial cultures are sold with around five to ten strains, much less than full blown kefir grains. They won't form grains, since the strains were hand-picked in lab by their function, and probably the grain-forming ones were left out.
Best Answer
The damage is not so much as to the kefir but to the consumer of the kefir if it was made in a reactive metal. Kefir, being acidic, should not be in aluminum, brass, iron or copper as they react to acid. Stainless steel is preferred because it is inert to the milk kefir is made from. However, when first made stainless was unknown.
Unless you have a real goat skin, about the best thing to make kefir in is glass. The "grains" are really a mix of yeasts and bacteria, and those can live in as much as a very light scratch in plastic. For this reason you should use glass to ferment it, and you can use a stainless spoon to stir.