What is this???

[tattedupabe]

This is our first experience with a saltwater tank(my 6yr old wanted a Nemo‍). The tank has been up and running since April, all of our levels(pH, ammonia, nitrates/nitrites) are all within recommended limits. We currently have one clownfish in there, a couple of snails, and a few hermit crabs. This lovely(yucky) display seems to have just exploded itself into existence while we were away over the weekend. What is it?? Is it normal? Should I clean it?? What should I do???

pick up some turbo snails