They are two different forms of the bacteria Pseudomonas aeruginosa (PA).
The term mucoid vs non-mucoid refers to the type of colony that grows when the bacteria is cultured; specifically the muciod produces a 'slime' or mucous coating typically.
I can't tell you the clinical means that one becomes the other; however, often the mucoid is more difficult to treat. This is commonly due to higher resistance to antibiotics, and a greater likelihood of colonization in the lungs making it difficult to erradicate.
It is possible to culture both at the same time, our son has for several months now, and as it turns out the non-muciod still responds to 'normal' abx such as Tobi, whereas the muciod strain is highly resistant to all but just a couple of IV abx.