Marco is a farmer.
The placement of slimes for later retrieval once they have bulked up their biomass photosynthetically seems like a kind of farming to me.
I am reminded of leaf cutter ants. https://en.wikipedia.org/wiki/Leafcutter_ant
The ants cultivate fungi, not photosynthetic organisms. But their colony is made of this fungal matter, just as you want Marco to build his fort out of his exosymbiotic slimes. Here is a leaf cutter ant nest structure made of fungus, leaves and ant secretions.
As regards symbiotic photosynthetic organisms, lots of creatures have these (sponges, corals, lichens) but all that I am aware of keep the symbionts - endosymbionts - safe within their own tissues. The ants have exosymbiotic fungi outside their tissues but they are guarded in the nest.
There are definitely symbiotic relationships between ants and plants, and the plants even make structures to help the ants, but it is not the same as the leaf cutter ants (or Marco) building new structures out of the exosymbiotic organism and associated material.
I think the problem with photosynthetic exosymbionts as you propose (the slimes) is they are not inside Marco and they must be in the sun, and so would be susceptible to being eaten by other things when Marco is away.
You knew this too and so made the slimes toxic to all but Marco - that could work. But what about conspecifics of Marco's species? Presumably there are others. Marco's cousin could show up and eat Marco's slimes while he was elsewhere. Maybe the Marco creatures have territories they defend from conspecifics - that would make sense.
I like this idea. The idea of a territory used to farm photosynthetic exosymbionts makes sense. It is so sensible I feel like it must happen but I cannot think of an example. Still thinking...
As regards building things out of your exosymbionts, why not? The leaf cutter ants do. You could build out of a tough, woody, lichen-like exosymbiont / slime.