Osteospermum like to be well watered. It seems from the info given you are watering well but the fungus in the soil though harmless to the plant is a RED FLAG to you that the drainage needed for such a watering schedule could be inadequate.

P.S Getting rid of fungus in any houseplant soil is a very hard task due to spore permeation in the soil, pot and surrounding environs. Though it does no harm to the plant, if it is an esthetic problem to you, my advice would be to dump the whole thing into a garbage bag and at the appropriate time, get a fresh start. Situations like these I call REFRACTORY situations- it never improves no matter
what one does.

PPS. Just viewed your photos. You do have potted plants. Why do you have rocks on top of the soil? They would be better at the bottom of the pot for drainage improvement. Rocks at the top are inhibiting surface aeration of the top soil layer hence promoting fungal growth.

Noticed you are using the same potting method with other plants(eg:your hydrangea) with rocks on top, the pot sitting flush on the drainage catch plate, and I bet ya no drainage material on inside the bottom of the pot. Lets improve this situation right now: place a few small pebbles on top of the catch plate, this will allow for complete drainage of the water out of the pot drain hole at the bottom; make sure that there is a layer of drainage material at the bottom of the pot(pebbles or smashed crock) before soil placement and make sure that there is not a miniature rock garden at the base of the plant. Elimination of this decor will allow for surface soil aeration. If these simple steps are taken you will see an improvement next season.

miniature rock garden :)
I was assuming it would benefit in the same way as mulch :) but I guess you are right, it is too tight space for soil aeration.
and there isn't any pebbles at the bottom of the pot.
Thanks.

Yes, you can mulch as a dress up to your potted plants and some do use pea gravel, stones and even flat colored glass marbles as surface decor but success at this is difficult or a hit and miss affair. I think a better method is to spread around a light layer over the top with Spanish moss which can be purchased in any craft store( they come in a variety of colors now). A light layer will not lower the ph of the soil beneath to harm the plant. Good Luck, and Best Regards!

Thanks Eclectic.