They have zero value to someone digging them out. The Fishtail Palms, most likely Caryota mitis, will probably go into bloom from the shock of transplant. Once a Fishtail Palm blooms, it dies.

David, these three and other palms go into a detached, two car, loft garage from December 1 to March 1. Definitely, NOT a palm for indoors, too dangerous. I had to saw off the tap root on these as they have a tendency to find cracks in the blacktop and grow into the ground over the course of the growing season. Remember, these guys grow in very arid regions so roots have a tendency to go deep seeking out underground water/aquifers (so deep pots are preferred). Big misconception is that date species from arid regions don't like much water, NOT TRUE. And some date species come out of tropical regions with monsoonal rains. Phoenix is a nice genus of palms but most are not for the house, with the exception of the Pygmy Dates (Phoenix roebellini).

Polar bear, I only have a handful of palms that get sooo big eventually that they get thrown out of the house...Phoenix, Washingtonia, Butias, and Tracycarpus. Sure, pot size will constrain their growth, as will a lack of optimal growing conditions...SUN, HEAT, FERTILIZER, and WATER (in the growing season). If you want something faster from seed, try Washingtonia Fan Palms (many of mine started from seed). I also suggested the feather palm Butia (they grow like weeds on steroids!). They are quite cold tolerant btw, so as long as get are freezing or above, they are fine in their Winter abode. Occasionally, they get top dragged out in midwinter to get hosed down, or I will cover their contains with snow to get some moisture, but they are largely dry, dry, dry, in Winter. I like them too, they don't seem to mind NJ's ridiculously high Summer humidity and rain, as do many Phoenix/Date palms. I see your palm is inside from your post. That us one reason you are seeing slower growth. They really are not indoor palms... They wanna be outside in the sun and other elements. Mine are out from about March 1 to early December. Adults are okay with some degree of frost and our growing season, even heat, tends to linger well through Autumn.

Ditto the above posters. Most Phoenix species have an issue with very high humidity and constant wetness. That's because many of them originate in semi-arid or Mediterranean climates where humidity levels are not sustained at high levels (the Canary Islands are rather dry remember). I once received a CIDP from S. Florida and it was covered in these ugly black spots. I figured at the time it was related to the humid climate and didn't make an issue out of it. Sure enough, it grew into normal growth and is now gorgeous. The discoloration you're experiencing won't revert to normal, but new growth will be more normal once your humidity levels drop. Good luck!