Diamond gobies

[tpate77]

I have two diamond gobies. The smaller one has burrowed and completely covered a lower rock and is staying in there for days the larger one is sitting outside keeping watch. They have been in the tank for six months and are always together but this is new are they breeding and the smaller one is laying eggs. It is not even coming out at feeding time. I moved some sand around this morning and both went to work building it back and the smaller one is gone again the larger one is at its post. Any ideas let me know what is going on