Hey mate. To me that definatly looks like a salt build-up in your soil gathered from unused fertilisers the roots aren't absorbing... flush your grow medium ASAP! Do this by taking it to a sink, turn on the tap and fill the soil up till it's completly drenched. 3 or 4x the amount water as you have grow medium. Then let the eccess water run out the drainage holes. The damage isn't fixable, but it's stopped.