I wonder if the antibiotic is killing off some of the bacteria in the tank.... The Mardel info sheet says that Maracyn 2 won't affect the beneficial bacteria (BB). But.....
Can you test the water parameters? Ammonia, nitrates, nitrites, pH?
The half life of Minocycline is 11-23 hours. Full treatment period is 5 days, and you're on day 3.... But I know that if my tank started showing white fuzzy stuff, I'd be unhappy! Also, some strains of Columnaris are resistant to Maracyn 2.
So.... two potential options for you to think about
1) Do a water change, to remove whatever the white stuff is. Then, continue the treatment for days 4 and 5.... Or
2) Change medications....
Personally, I'm concerned that Merlin has NEW fuzzies on him. Due to the speed at which Columnaris can grow, I think I would opt to change meds. (Between the fuzzy tank and the fuzzy fish, I wouldn't be happy!).... You could try one of the other meds that treats Columnaris:
- Kanaplex (Kanamycin)
- Furan 2 (or) Bifuran (or) Jungle Fungus Clear (all three products contain the same two furan antibiotics)
- Triple Sulfa
Maracyn 2 Info Sheet and Directions
Minocycline Fact Sheet