The windscreen washer system can become clogged with algae and other bacteria formed from a combination of water and screenwash residue. Any body of water left long enough will accumulate unwanted detritus, and any dirt within the system will eventually end up in the washer bottle. Mixing different brands of screenwash can cause sludge to accumulate, so we would suggest choosing a good quality brand and sticking with it. To avoid this happening in future there is no substitute for emptying the system and flushing it through with plenty of clean water, although this can be a time consuming task, so you may wish to try a cleaning additive which can be added to your washer fluid.