Thanks for your comment - the walls were modeled with PU foam and were covered are a couple of layers of a mix of cement, coconut coir, and vermiculite - following recipes for Hypertufa. Looks like rock but doesn't have much weight and supports moss growth.
Its actually a thermometer. I have a larger 10-20 gal heater but I hid it behind my filter and the Val. The thermometer detached from the front of the tank and was floating around when I snapped the pic. I wish I could find a shorter one too!