Yep algae phase is pretty normal in a new tank like yours. Overfeeding is the main culprit for the increased nutrient levels. Thing you can do is lower the amount you feed each time, run gfo, more frequent water changes ect...
Believe it or not the algae, looks horrible, but is actually doing you a favor in eating up the excess amount of nutrients in your tank. Just manually remove when the tufts get long enough and try and keep your nutrients low. It will take many months for it to go away once you have nutrients under control, so be patient.
And yes that is quite normal for pincushion urchins, they will pick up what ever they can carry and use it for camo. Mine carries around a large bunch of my macro algaes and looks like he is a moving plant.