Yes, the sponges need to be cleaned up often or they will cause overflows. When the sponges get dirty, they start to obstruct the flow of water. If you obstruct the flow of water, the skimmer will intake more water than it can push out and it will cause to overflow.
Please remove the sponge and give it a good cleaning and that will fix the issue.
The bottom sponge acts as a bubble trap so you are able to remove it without compromising the filtering functionality of the skimmer but you may end up with some micro bubbles making it back to your main display system. You may not also so you will have to test to see how it does in your system.