Often, an ooze can go for a very long time between meals. When faced with prospects of starvation, some oozes adapted a dormant stage – and of those, a better survival trait was a mimicing dormant stage.
The ooze goes dormant and assumes the characteristics of the surface it is on. Like a bacteria spore, it can last nearly indefinitely in this state. How long has it been since anyone has disturbed this tomb?
But once disturbed by movement near or upon it, the ooze begins to awake from its dormant state. Allow 1-3 rounds of disturbance to rouse the ooze. Then in a progression of 4 rounds, the ooze returns to its normal state.
So, it goes like this:
The dormant ooze at the bottom of the pit appears to be the same material as the stone floor beneath.
Landing on it, the ooze is as hard as the stone the players were expecting. But then it wakes up.
In the first round, one character or two (the dwarf's stonecraft in this example) may notice that the the floor shifted beneath them, subtly.
The next round, several characters feel like they're on different boats, rocking on the ocean.
The third round, the ooze begins loosing its mimicry characteristics. While not yet corrosive, it now looks its natural color and the characters see this transformation of the floor beneath them.
On the fourth round, it starts eating them from the boot up.
Choose your favorite ooze or roll one up.