NCAA Rules

Try and relax. You should be fine. Lots of players also attended pro tryouts.

The reason your NCAA certification is still under review most likely has nothing to do with your participation in those tryouts.

Any prospect is allowed to participate in a tryout with a pro team, as long as the prospect "...does not receive more than actual and necessary expenses to participate."

Just really worried. My academic part is already approved, I really don't know what else could possibly be the hangup with this. Received nothing from this, just did it for fun.

Most top college recruits attended pro tryouts. That is no violation. If it was a violation virtually everybody in the college world series would be ineligible.

If your academic certification is already taken care of, then the holdup might be in the way you answered one of the questions on the Amateurism certification portion. You may want to go back and review the answers you provided to those questions.
Did you indicate whether you have an advisor?

It also could have nothing to do with you specifically, but the Eligibility Center may be conducting a comparison of your amateurism answers with other baseball prospects of a similar demographic (from same region of country, other signees of that school or conference, etc.).