I am not a big fan of Mojo, but I don't particularly like the Hippies either. Had I been in Mojo's shoes, I would have been just as pissed to be yielded and particularly to have rumors started. So Eric is flirting with Monica? If Joseph doesn't like it he tells her. And what is Monica supposed to do? Tell Eric off and risk making an enemy out of him? There are still in a competition. You don't want to piss off anybody.
As for the use of a yield. It was a real waste. I would have used it on the Frats, who were a little further behind and who are the hippies real competition. See how fast the frats caught up to the hippies in the river?