He decreed that they not only had to make bricks but also to gather the raw materials for the purpose. In his question, Moses was admitting that G-d's will was incomprehensible to him. Nevertheless, the next time G-d commanded Him to do something, Moses obeyed with his whole heart. True bitachon doesn't mean not having questions. True bitachon means resolving in one's own mind that the answers are there and that they are good, but that they may not be known to us at the moment.
True questions are an admission of our own smallness, of our limitations in comprehension.
Questions are not a lack of faith. But to think that the question is the answer is a lack of faith. No one in the middle of a story is able to see the end of the story. Bitachon means believing that there is an end to the story, and that if we could know the end we would have no doubts now. Often a person can seem to fail in the physical world, but be victorious in the spiritual world.