Where did you get it from? Do you know what type of system it was in before, or under what lighting? What kind of lighting do you have? Any tank parameters you wanna share i.e. nitrates, ph, calc, alk?
I really do think it's an encrusting monti, and the rainbow variety at that. I had that same piece a long time ago under a 4 bulb t5 75% up in the tank under moderate/high at best flow and I STILL could barely get the rainbow color in it. I would give it a good amount of random intermittent flow and direct lighting if your parameters check out alright.
Edit: yours is also doing exactly what an encrusting monti would do under either too low of flow/light or both.