This hand pre-flop is kind of marginal, we can probably muck it and not feel bad about it. On the flop, we hit the underfull, and against these players we'll be looking to get stacks in (against smarter competition I'd play it more cautiously, because you really could only expect action from QQxx, but here at .05/.10, all trips will get it in here, AAxx will probably play, etc). There's no way you'll get stacks in if you don't raise on the flop or turn. It's nearly criminal to slowplay this much in PLO. Srsly.
As played, villain could have 99xx for a better full house (QQ/JJ also give him better boats, I'd expect a raise on flop/turn with those hands though), but he could also have a straight that he thinks is good since you've really underrepped your FH, so we should at least call. Any read we have would sway our decision towards either flatting/jamming.
If he did have 99xx, then I hope you understand how terrible it is to slowplay here and in PLO in general. We really don't have a ton of equity with our FH, so we still can't afford to give free cards, and almost never are we so far ahead that we should slowplay. Next time, play it fast on the flop, there's only one hand that is straight up beating us at that point.