The prerequisite course starts in about 2 weeks [1], so you can still take it if you want before Principles of Reactive Programming starts, you'll be able to see what parts you feel you can skip and which ones are useful. There are other quick introductions to Scala you can take [2],[3],[4] to get a feeling of it.

[1] https://www.coursera.org/course/progfun

[2] http://www.scala-tour.com

[3] http://scalatutorials.com/tour

[4] http://learnxinyminutes.com/docs/scala

It starts in two months so could give Martins books a read. And I think that would be enough. Some of the lectures were based on his book and others on SICP.

If you're well versed in Clojure and FP principles, the only difference for you would be in picking up Scala syntax, in my opinion.

you won't have trouble. that said, the first Scala course is excellent independent of its use of Scala because of its fun/challenging homework assignments