yea the jellyfish is harmless. i've been there several times as well. the ones at the jetty are huge and deadly. its in sharkteef's pic. the exact same one i saw. the ones on the island are small and not deadly. even if it stings you, marks wont appear. I've learned much about jellyfish and if you ever get stung by one, you can either pee on it for immediate effect, or reach back to shore and pour hot water over the stings. It reduces the burning sensation effects faster than any antiseptic lotion.
Perhentain and Redang departs via Kuala Terengganu jetty. The Merang jetty which is still an hour plus away from dungun town. Kapas is a spoilt place. If you go to kapas or rhu, you will find that all the good things have been destroyed. Many have gone there and complained such as tioman itself. I have to agree with sharkteef that tioman is a commercialised area hence the beauty that many seek isnt there anymore.

i didnt know anyone would find out about tenggol island. looks like i was wrong.

at the island, yes, its harmless. the resort operators will tell you all about it if u ask.

easily bump into 1 if its on a warm weather day.