Tracey and Cobweb5 years, 11 months ago
You might find the FFU guide a bit easier to follow:
http://www.freedomfairies.com/bottom-dwellers.html
But to answer your other questions, "uncoloured pots" are used rocks, from the pondweed. When they are used, they lose their colour, and it's these you need to get the corals planted.
The mushrooms you need depend on the underwater myth you need, and need to be RTH so you'll need to use the Rainbow Rocks (from completing the Rainbow of Fish) to get them RTH immediately in order to get all of the corals you need in time. You can use a fully grown supershroom to deter some of the mermaids once you have all of your corals in place, if you like, but otherwise fully grown mushrooms have no effect underwater.