yeah looks like the very beginning stages of HITH to me, but since you caught it so early as long as you can find the source and fix it, it should be able to heal in a little while and not leave any permanent scarring
and Goestonemoa, if you still have all those oscars in a 90g then they probably do all have HITH, there is no way you would be able to keep nitrates under control in that size of a tank with that many larger oscars even if you had like a continuous drip water change system or something