The information about today's challenge is in the post you quoted, as well as in the second post of this thread - but feel free to ask more detailed questions about it, if you wish.
In relation to whether or not it is "open to the public", you have to enter the competition in order to be able to participate in these daily challenges. However, participation is free (assuming you would be betting anyway), quick (just make an entry post in the Registration Thread, and you're in), and painless.

Hmm challenge will be edited / changed every day like this quote. Ok I understand still waiting for the next challenge
NEW DAILY PRIZES CHALLENGES
Previous challenge: Parlay won at highest odds (if no one wins a parlay, the user with biggest ROI% on that day will win the challenge).
Next challenge: The user with the highest total daily winnings percentage (ROI% on that day) will win the next challenge.
Daily prize winners: (winners will be posted here in order.)
#w1: (GrandeMar); #w2: (Miyslovenic);
Hmm, do you mean, today's challenge? That was part of what FrueGreads was announcing: the winner of the previous challenge (the parlay challenge), and today's challenge ("The user with the highest total daily winnings percentage (ROI% on that day) will win the next challenge"). Perhaps he should have worded it differently: instead of "Next challenge", it could simply be called "Today's challenge", or something like it.
At any rate, the challenge he announced in his last post is the current challenge, and it will last until 23h UTC today (when we will announce the winner, and a new challenge).