Monster Kelp are plant-like organisms which consist of a long, thick stem and drooping leaf-like growths. Monster Kelp is primarily black in coloration, accented by red or green edges around their 'leaves'.
Tips
Combat
Monster Kelp present a large and mostly-stationary target, and as such are fairly easy to kill, dying in ~3 Lanco hits. Monster Kelp writhe around when hit and cannot grab Slugcat while doing so; thrown Spears can be retrieved during this period. Targeting the base of the stem can make it easier to retrieve Spears, as it remains the most stationary while the Monster Kelp is writhing. Monster Kelp that are submerged in water or growing from walls or ceilings may be harder to retrieve Spears from and as such may require multiple Spears to kill.
Evasion
Monster Kelp can be stunned long enough to get past by throwing a Spear, a Flashbang, or alternatively a Rock (although this stuns it for a considerably shorter time) at it.
It is possible to distract Monster Kelp with objects that have player collision, such as Blue Fruit or hydrated Bubble Fruit. Throwing one such object near or at the tip of their stem may prompt them to grab the object and retreat into their den with it, granting a decent window of time to pass through.
Stealth
It is possible to sneak past Monster Kelp by moving slowly. This is possible on both land and water (although the water must not be disturbed by something such as a waterfall) and can be executed by tapping the move button a bit at a time. If this is performed successfully, the Monster Kelp does not react to the player.
Abilities & Behavior
The base of the Monster Kelp remains stationary, but their flexible stem can move and attack other creatures. Monster Kelp remains still while no creatures are in sight. Upon detecting prey, Monster Kelp stretches towards them and follows their movement with the tip of their stem. If the tip makes contact with their prey, the creature is dragged into the Monster Kelp's den and killed. Monster Kelp hunt almost all creatures.
Monster Kelp may also attempt to grab objects that have player collision (e.g. Blue Fruit) and drag them into their den.
Senses
Monster Kelp have movement-based vision which takes the form of a full radial view from the tip of their stem. They are completely unable to hear.
Taking Damage
Upon taking any amount of damage, Monster Kelp writhe around before either retreating into their den if the damage is lethal, or returning to an active state after a period of time determined by the damage taken. While writhing around, Monster Kelp are unable to grab anything. While not being damaging to Monster Kelp, Flashbangs are able to stun them for a considerable period of time.
Monster Kelp vary in stem length, stem width, 'leaf' density, and individual 'leaf' length. They can have either green or red 'leaves', but green Monster Kelp are exclusive to Shoreline.
A medium, thin stem, and generally medium-length, red 'leaves'
A long, thin stem, and generally long, red 'leaves'
A medium, thick stem, and generally short, green 'leaves'
A short, thin stem, and generally short, green 'leaves'
Trivia
Monster Kelp cannot be tranquilized by Spitter Spiders' Dart Maggots.
While real kelp is only found underwater, Monster Kelp is often found on land.
Some Pole Plants have a chance to lineage into a Monster Kelp when killed.
If the player is grabbed while carrying a Lantern, there is a chance that the Monster Kelp drags the Lantern into their den instead of Slugcat.
Developer commentary on the design of this creature as well as that of Daddy Long Legs can be found here.
While Monster Kelp is usually found growing out of the ground, there are various instances in Shaded Citadel and Subterranean of it growing out of the walls or ceiling.