Dealing with algae can be a battle.

We had a Cooper S cabrio which suffered, and tended to use Autoglym (a 2 pack shamooo and protector) but the algae came back after a few months. House has many trees around it, so no surprise. Used to shampoo hood and scrub algae with a nail brush to get deep down, but was never 100% pleased with end result, looked patchy when dry. The protector needs to be kept off windows too.

Others I know have tried a diluted solution of Milton/water, around 1:5 I believe, before a second stage proprietary treatment, while others swear by Meguiars Convertible & Cabriolet cleaner. Never got round to this before passing car on to daughter.

I suspect treatment is something you will need to undertake every 3 months to keep on top of it.

Milton is supposed to work and some people mix it with the hood cleaner water.

However the single best precaution I took when my MX-5 was living outside was a half cover. It keeps the bird carp off the roof and the rain out of the hood drain which stops them bunging up with leaf bits and general muck. Just have to be careful that the car is clean where the cover touches it - I used to wipe round the tub with a microfibre cloth using waterless wash. Never saw any damage.

I'm a bit of a connoisseur of half covers. I'd avoid the ones with elastic chin straps - they don't stay on well in high wind and the elastic loses its virtue quite quickly.

I suppose if you use the car daily a cover might be inconvenient, but you don't really need it in summer.