When I have feeded with sugar bees at autumn, sugar liquid get fungus if there is a little bit dirt in the case. Dirt gives necessary nutritients to bacterium or fungus growth and it happens what happens. Pure sugar does not allow the growth.
So, clean very carefully the bottle, all parts an use pure sugar liquid.
Bee can transfer dirt to the feeding case and fungus get a foot step from dirt.
Also you must wash the case if you notice something.