A fluorescent yellow colour is usually associated with a water borne bacterium, Pseudomonas. It is not considered a pathogen, but a spoilage organism, and produces off flavours. Could be due to poor sterilisation, or poor water supply, either in milk production, or cheese making. Sterilise all equipment, and rinse with cooled, boiled water before use to eliminate the organism. If it persists, it could be the milk, which should be pasteurised before use.