Split main stem turning brown/tan/grey, mold?

Covering/repairing the split with tape can encourage mould/fungus to attack and it is best to leave the wound/split uncovered while the plant repairs itself. It should be ok, the lack of green colour is most likely due to being covered with the tape and the chlorophyll in that area "dying". Leave the split in the open air and your plant will recover soon enough, but don't ever cover any wounds in the future with tape again, just leave it be, just like in nature. The fact that the application of tape has worked for you in the past is pure luck, tape will cause an overly moist area that mould or fungus love. Hopefully the brown area is not mould, but if it is, it should die now that the wound is in dry air. If you are really concerned with splits in the future, using rubber covered wire to wrap the area in lose spirals is far better, as it allows air exchange and a dry area around the wound. In fact, using wire and a splint of some sort can repair a stem to full health, whereby there is a "scar", but the plant can support the branch/stem that was previously broken and return it to being fully functional and self supporting of any weighty flowers further down the track. Hope this helps, ...... Organoman.