soooooooooo wtf is this? and why does it creep me out

[MKLKT]

So I did a 20% water change today and it apparently signified a fiesta celebration for the micro brittles in my tank. Check out all the arms in the picture. There were some riding the polyps of the various corals in my tank too, it was hilarious. The picture doesn't really do it justice, usually you can only see maybe 30-50 out and today it was in the hundreds easily.