I dont think washing up liquid will do it, if its powdery mildew but preventative measures may be the best way forward.
This type of mould thrives best in dry shade, so keep it light and airy with plenty of ventillation.
Remove any dead or decaying plant material.
Occaisionally give the leaves a fine mist spray with water.
On mild rainy days, the plant will benifit by being put outside for a while.
Keep it moist but not waterlogged, the water should be seen to drain through fairly quickly.
Hope this will be of help, Good Luck Tbird+