I had kept mustard oil cake dissolved in water in order to make fertiliser tea. But a whitish layer appeared over it after three days. Will it be safe to use that tea on my potted plants?
The agricultural industries are constantly looking for ways to make better use of their otherwise waste materials. Normally when we think of mustard/canola type cakes and meals we think in terms of their protein content and therefore their use as animal feed. After this kind of use the secondary waste product can be applied to horticultural and agricultural uses.
Research seems to indicate that great care is necessary when using oilseed cake/meal with plants. Any time you make tea, there is bound to be some separation of the meal components. Tea extraction could concentrate the more potent parts of the mix to the detriment of the soil, so great care in dilution is needed. North Carolina State University Extension did some experiments with meal from soybeans with mixed results. It seems that while it can be a source of N,P,K it can suppress germination of small seeds, which may be good in an already established potted plants. Also see this previous question related to compost teas.
The problem is that unlike using a well known categorized and analyzed product you are taking a risk applying such products directly to plants that you value. What is probably a safer route is to apply the tea as fertilizer to plants that you do not value and then compost those plants to mix in with the soil you grow your more valuable plants in.
The only way of making mustard cake tea for use as a fertilizer that I know about involves adding a fungicide powder after steeping the broken up mustard cake in water for 3 days; the formation of a white (presumably fungal) film on top prior to the addition of fungicide is likely not unusual, but the mixture should be kept for no longer than 4 days maximum prior to stirring well and mixing in the fungicide. Unfortunetely, I don't know the ratio of fungicide powder to the mixture, but there are a number of videos on this subject online, usually in languages other than English. My knowledge of this subject is otherwise non existent, never having used it myself, but searching on line for 'how to make mustard cake tea' should bring up some results.