Bry, your post was the stimulus I needed. I cut the end off of both canisters and pulled out the filter elements. Both were getting plugged up with algae but the secondary looked almost totally plugged.
The primary filter is the one in the front (LFP815FN). The secondary (LFP816FN) is to the rear and has 100% of the element covered with the goo.
Looks like the algaecide that Newell added did its job, as did the filters. I will probably add some additional algaecide just before I have my next trip planned and carry a pair of spare filters. I may carry two of the secondary filters since they do seem to plug up worse than the primary.
Nasty!!