Marks on live rock

[Oarfish]

I notice this pattern on live rock, seems like something had chewed on the algae and leaved marks on it. What could do such things? Urchin? But I did not notice any around. Any other possibilities?

On the other side there's some slime and some sand-like stuff (I don't have sand, my tank is bare bottom). This rock is partially above water, and the slime is above the water level.

[Harlequinmania]

Do you have sea urchin or abalone ? Most likely is them

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[Oarfish]

No, I don't have any urchin or abalone, and so far I haven't spot a hitchiker (though I have one small fish missing, never to be found). There are 2-3 asterina starfishes though. Is it possibe that they are the one leaving these marks?

[hazelnut]

urchin don't leave marks like this, they will spiral around and leave a trail of line. not big patches.

might be a snail. when lights off In the night, turn on your torch light and check.