A guide for the boss shown in the picture. This looks like a wall of text but I've tried to keep it as short and informative as possible.
Included: Form requirements and the path to get to this boss.

Tags covered:
-Crafting: External wiki link to Aquaria cooking recipes
-Loot: the item gained from the boss on success; info: random loot drop on successful repetition after gaining the item
-Walkthroughs: how to get to the boss; what to do

Simon says mini boss
Difficulty: depends, hard for the most I guess

Requirements:
Beast Form
Fish Form

Grants:
Additional cooking slot. This allows to make use of cooking with it's potent recipes to it's full extend without having to visit a place that supports a 3rd cooking slot.

How to get to the Simon says mini boss: (See also: Map section, below)
To get to that mini boss you need Beast Form, Fish Form and use a Travel Turtle within the Arnassi Ruins. The Arnassi Ruins are there where the Rock Crab boss can be found.

Start: Home Waters
Destination: Arnassi Ruins
1) Home Waters map: use the eastern exit at medium map height to the Open Waters map
2) 1st Open Waters map: proceed east to another Open Waters map
3) 2nd Open Waters map: with Nature Form you can use a shortcut plant in the upper right area where you just entered the map; you'll need to get to and use the soutern exit of that map, leading to yet another Open Waters map
4) 3rd Open Waters map: 2 paths lead to the east; use the bottom one that leads into a cave that isn't a dead end but has an exit at it's bottom right corner and use that exit to get to the Arnassi Ruins
5) Arnassi Ruins (Crab Boss area): the southernmost cave has a statue on it's right side behind which a Fish Form path leads northeast to a Travel Turtle that'll take the player to the Simon says boss
Note that this Travel Turtle is not connected to the network of other Travel Turtles so you'll have to use this specific one to get to the boss.

Description:
-no combat, event based boss
-cancel your current form so that the song circle is usable
-get right next to the boss in a distance that allows free view on the boss's tentacles even when the song circle is shown
-click the boss with what would be your attack button in Energy Form
-observe

The boss will start showing a single tentacle's color and play a sound along with it, let's call that a variant and all variants in their shown order a sequence.

Objective:
The player has to repeat the shown sequence each time the boss finished showing one.

Failure:
Picking the wrong color counts as failure, accidently transforming while solving a pattern counts as failure (unless the boss already shows a new sequence), taking longer than 4s per sequence to solve one counts as failure.
Each time the player fails the boss growls angrily and you'll have to interact with the boss again to start a new, random sequence at 1 again.

Success:
Each time the player does a sequence right the boss shows the same sequence and adds +1 new variant to it's end. The player then has to repeat the whole new sequence that was shown. After having solved the final sequence, consisting of 8 variants, the whole thing is completed with success and an item is dropped. This item grants the player access to the 3rd cooking slot, if picked up.

Gibberish:
The human brain has issues with memorizing more than 7 things and there are 8 things to memorize here. It helps if geometrical forms, logical patterns or repetitions are involved. It might also help to cut a sequence into parts. Whatever helps you to memorize a sequence in a way to recall it fast and reliable also helps: memorizing clock-like number arrangements, angles, colors, letters, cardinal directions, gamepad-like directional arrows, aquaria icons...
You can also hit the menu button after the boss is done showing a sequence and write the +1 variant down but you need to do the whole sequence flawless within less than 4s nevertheless once unpausing the game.
To reduce distraction turning off the music also helps. After obtaining the 3rd cooking slot it's possible to repeat this, which grants a random item upon success.

Value:
Depends on how much you make use of cooking in this game and gather ingredients for that. With the 3rd cooking slot unlocked you no longer need to visit home to make use of even the most powerful receipes. For people that integrate cooking well into their gameplay this'll be a massive convenience feature while for the others this'll allow them to maximize their amount of Sea Loaves produced.

External wiki link for a recipe list: Link[aquaria.wikia.com]